I just use my regular buttercream (crisco - no butter). I've shared this story a few times, so my apologies to those who are tired of hearing it 
but I made a dummy in April '06 (for a wedding) and just cleaned it off a few weeks ago. The BC dries hard but I just run the knife under it and it pops right off. No mold or "spoilage". It sat in my display room for about 10 months.
As far as doing a dummy w/ the basket weave on the side, would you recommend RI or BC? It will be in a display case, but will need to last awhile.
I make mine with home made fondant, I eliminate the glycerine and put a table spoon of cmc per 2 ponds sugar.
cover the styrofoam with saran wrap, the moist with a little bit of water and cover with the fondant.
You can decorate with royal icing.
They will last about a year, all you need to do is dust.
Thank you both for your suggestions! I recently did my first one with fondant (which came out fine, thanks to CC), but the basket weave had me stumped. CCer's are such good, helpful people! 
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