I have some recent experience with that expanding foam stuff in a can ~ no dress form experience though. when the foam meets resistance, it goes another direction, so i can't imagine it distorting if your duct tape is thick enough and firm where you need resistance. also allow the foam to have an escpe route as it expands ~ shoot in from the top, say, and let it expand upwards. the foam also might escape through micro-holes in the tape form, but just let it spew out and don't mess with it until it is dry - then you can easily shave it off with a blade. no scrapey-wipey when wet = very messy!
ok that's all i know.
OH WAIT wear gloves & crappy clothes if you use the foam ~ it is sticky and difficult to remove from skin and clothes.

I have a fair amount of experience with polyurethane foam, and I wouldn't recommend it. The foam needs a rigid mold to expand against. Foam is dessignated by the amount of pressure in poiunds/inch they excert on a mold. So when you buy foam, you can get 4lb, 8, lb, 12 lb etc. This is a lot of pressure to apply to a soft form, and considering that duct tape has a fair amount of stretch, it will most certainly distort your form.

When I tried it, it didn't work so great. When it expanded, it seemed to have split the back seam (where I had cut it off and then taped it back up) and, since it dried hard, I couldn't reshape it. It also didn't expand nearly as much as I thought, so I stuffed the rest of it. I prefered the stuffing method, in the end. After it's all done and mounted, you can then recheck your measurements and adjust if needed--for example, the waist ended up too wide so I cut duct tape to my waist measurement and cinched it in. The added weight of the foam also has bent the PVC pole mount over time, so I have to remount it.

Hi Stephanie!

I tried to make one of these forms with the expanding foam, and it wasn't pretty. I think the form itself needed to be lined with newspaper, rather than left with the saran wrap I used to protect my body. When the foam dried, it pulled the saran wrap inwards and distorted the form (I think if it was lined with newspaper it may not do that, but I haven't tried it yet). Polyfil might work better.