A glass shop should cut you panels for cheap..Just bring the tank with you and tell them what you are doing. The last time I partitioned a 29 gallon, they charged me 30 bucks.
Silicone will stick to acrylic somewhat, but long term isn't not going to hold up like it would glass. Not to mention, unless you use thick pieces, it's going to bow, and that's what makes it slip away from the glass. I would use glass.
I would want the biggest sump I could fit in the stand or allotted space I had. A 55 would fit. A 20 is very limited.
The important question is, what are you going to use this sump for? A skimmer? A refugium? If so, you need ample space for these components.