On the cyro website, there's a calculator for aquarium acrylic thickness; it's pretty conservative. Plugging your numbers in without a top brace it said .85" for 20" high and 1.35 or something like that for 30" high. This means you're supposed to use the next available thickness, which in this case is 1" for the 20" tank and 1.5" for the 30".
I've made sumps out of material quite a bit thinner than the cyro calculator says, but I think in this case I'd avoid anything under 1/2". With a top it said .55", which means you could use 5/8" (.59") So maybe that's a good thickness to try without the top if you want to stay sort of in the ballpark of the calculator but not be quite as conservative.