I've got a design for a rather simple puppet but I'm a bit stumped as to the best material to use.
It's just a singing pickle with a very basic pull string mechanism as illustrated below.



This is my first attempt at anything other than a big mouth puppet and so far it's going well, the mechanism works as it should.

However, I'm not sure what material to use for the outer facade.  I imagine there's a good foam that I could just fasten/glue to the upper and lower jaw parts and the mechanism would just pull them apart to open the mouth, but the foam I have doesn't quite have enough give to do that. (It's just the green stuff from the local fabric store, I suspect if they had something lower density it could work, but they don't sell it there anymore)

I know there's all kinds of different foams out there, but I'm a bit hesitant to order anything yet because I'm not sure what would be best.  Or is there an entirely different suggestion that may work better that someone has?

Well, the design looks great, kinda jose jalepinio 'on a steeeek', i like the idea of a singing pickle!! maybe you could build the general structure of the picle with the foam you have, but leave a small wedge section space under the lower jaw plate, then cover the entire thing in a greenish fabric covering the gap aswell and then when the lower jaw opens, there is enough space to enable the mouth to open. infact if i remember correctly, there was someone on P&S that made some big cactus puppets using this method, except it was some of those pool noodles, so maybe you could use insulating tubes for radiators and build up some nice pickle bumps with funky foam or polyfoam, then cover the same way as i suggested before!!! if you have ever seen the dancing hotdogs from 'puppets up (the brian henson improv puppet thingy) then you will get some idea of what i mean, they work great and have so much personality, and are basically what you are aiming to make. i think also that a little trigger on the control, post would help aswell with the opening of the mouth instead of the ring you have in the design. good luck with this, i hope my input has been helpful, id quite like to have a go making one myself!!
Ben.

Do you want a flat puppet with a moving mouth, or a dimensional oval shaped puppet?
Flat could be made from foam core (Billy just did  a post on the good stuff) or Sintra.
Otherwise a hi density 1/2" foam should work great.  You could dye it, spray with floral spray, or cover with fabric.

I ended up using the suggestion based on the cactus puppet, using the foam with the area below removed and wrapping it with fleece.

Here's the prototype puppet and it didn't turn out too bad.  It's missing bumps and eyebrows, but it works.

This reminds me of a poem I wrote in 7th grade.
 
           Pickles

Pickles, pickles on a stick!
Pickles, pickles make me sick!
Pickles, pickles, oh how I hate pickles!

Cool, looks great so far, actually, it already looks alot like the picture. glad that i could help, im looking forward to seeing it finished and in action!!
and i like your poem Aaron, it evokes the feeling of never wanting to eat picles ever again!!! lol!!

Can't wait to see it in action

Looking good!  In regards to the issues you had with the trying to do this with the foam in the first place, the secret when you are building in foam is that you need to pattern or create the opening of the mouth in the open position not the closed position. So instead of stretching the foam you are compressing it. This is the same concept that is used when a puppet builder sculpts a puppet out of a block of foam.  The mouth neutral position is open and when the hand goes in it closes the mouth.

Looking good!  In regards to the issues you had with the trying to do this with the foam in the first place, the secret when you are building in foam is that you need to pattern or create the opening of the mouth in the open position not the closed position. So instead of stretching the foam you are compressing it. This is the same concept that is used when a puppet builder sculpts a puppet out of a block of foam.  The mouth neutral position is open and when the hand goes in it closes the mouth.

I've learned this for my hand puppets, but for some reason didn't think to apply it to this puppet.  It makes sense, given that the mechanism is really just a tiny hand hidden inside the puppet.  For the final versions I'll certainly make note of that and alter the design accordingly, I may need a stronger rubberband or spring, depending on what the mouth is made of, but I believe that will improve on the design.

As far as prototypes go, I'm pretty happy with it, hopefully the next go through turns out right.  I need to make four of these for my project, and even though it's turned out better than I expected considering how rushed it was, this first one just isn't going to be stage worthy (if only because I used all purpose thread instead of monofilament, because I didn't have any on hand, and it's bound to wear out soon). 

Another thing to consider that is going to wear is the rubber band or spring that you use.  Try to build so you can have access to replace this.  Lot's of times it seems that a rubber band works better and you can control the amount of resistance you need by doubling or using different thicknesses or even more the one, but you have to leave a way to get into where the attachments are for them.  If you are going to use these a lot or over a period of time both springs and rubber bands can wear out.