Hello & Welcome!!
To get the soap out of the pipe, I use laminating plastic, you know like they do posters in. I get the trimmings from the office that I work in, but if you go to any shop that laminates things and ask them if they would give or sell you some offcuts.
I've tried baking paper, and while it releases from the soap really well, the oils from the soap batter will soak thru it and stick to the PVC, making it really hard to get out, even if you put it in the freezer. I'm actually going to try both with my next batch - baking paper inside the laminating plastic, inside the PVC, just to see if it makes any difference. Just waiting for oils to turn up, why does the mail have to take so long... :cry:
As for keeping the PVC upright, if you cut the base straight across and use a pipe that is not too tall for it's width, they stand up by themselves no worries, as long as you are a little careful when pouring. The ones I've just gotten are about 33cm tall (roughly 1 foot) by 9cm wide (not quite 4 inches). Tho, I have found that 9cm is maybe too wide, I prefer the bar to be a little smaller. If I can get some pipe that is 7.5 cm (3 inches) it would be better.
I find that the beauty of the PVC pipe is that it can be as big or small a mold as you like. If you want a small 'test batch' sized mold, just cut it shorter. It is also cheap, which means that I have much more $$ to spend on fun stuff, like colours & fragrances and additives, and the list goes on, and on.....