Question about a diy vacuum chamber

So I'm going to be building a vacuum chamber to purge bho in and am wondering how thick of plexiglass/acrylic sheet needs to be used? I can only find stuff .118" thick and its pretty pricey for a big chunk, but i know i can layer it up to make it thicker. So now the question is, how thick does it really need to be to not break, I mean in a 10" circle how hard can the vacuum really pull the acrylic down? I kind of see people using what seems to be anywhere from about 3/8" to a full inch thick, and I think a full inch is definitely overkill! Especially with me only having a 10" diameter pressure cooker pot which I'm going to be using as the chamber. Also does anyone have a good link to a good read on how the process of purging actually works (not a how to but an actual explanation of whats happening when we purge and why we do what we do) I just think that will help me understand more of what I'm doing when purging. Instead of following a "recipe" of do x + y and get z.

stay stoney,
TooBakedd