I built a sump using a 20H for my 60 cube because I wanted to maximize the sump volume and a regular 20 was too big for the stand. I ended up using a sheet of .25" thick acrylic I had sitting around for the baffles. Having said that, I would not use anything thinner than .125" since you're asking your silicone (strictly a mechanical feature) to hold a lot of deflection with anything thinner. This is made worse when you remember to leave a gap between the glass and the acrylic to account for the acrylic expanding out (I used a 1/8" gap per side). Lastly, be very generous with the silicone with whatever thickness you go with.
Nope, did this exact thing with a buddy on his tank/sump 20H. It "works" but the baffles bow A LOT. One actually broke the glass/silicone seal after a few weeks. Ultimately, we bought another 20H tank (Petco$1/g sale) and made a sump with glass baffles.