So I did a bit more testing on foam core prop stuff.
First off, the original test part. It was (as you might recall) was done on a small hunk of foam with two layers of cloth. Now, the two layers was enough to make ot pretty sturdy, but I decided to add two more layers: and that thing is rock solid now. I think I could get away with three...but overkill!
Anyways, the reason I bring this up is that I was able to add more cloth to the test piece without ruining the part (ie, making more ripples in it inadvertently). This means that if you put too little onto the foam core, then melt out the foam core, you can still add more to the part without ruing the shape you may have had with the foam core there!
Second off, I did more testing on melting the foam core. I wanted to try and cheapen the cost of the melting by seeing is a diluted mixture would still work. Short answer: it doesn't. I first tried a 1:1 water/acetone mix (which I have in a spray bottle for Percy [my 3D printer]). This literally didn't do anything but make the room smell like nail polish remover and make the foam wet. Next was a 1:2 water to acetone mix. This did a bit more, but not enough to actually be useful. So, Short and skinny of it: you'll be better off sticking with a 100% acetone mixture.
Which brings me to the last point on this last run of tests: straight acetone will get...cloudy. I'm sure that you can only use so much acetone on the foam before the 'melted' foam inside the acetone ruins the acetone's ability to eat at the foam. However, at this time, I don't think this is a massive issue as the acetone pot I've been using is a 8oz jar, and the foam bits dissolved inside that pot look like a pinch of blue sea salt. I'll probably do more tests on this to see just how much foam I can melt into an 8oz jar before it [acetone] can't melt no more.