Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Source: Look locally, first. I do not know if they are nationwide, but there is a store called Tap Plastics in my area that carries acrylic, lexan and other 'plastic' materials.
No, silicone will not adhere well to acrylic. Maybe well enough to work inside a sump, where a bit of a leak is not a problem. The Window and Door will stick well to very clean glass, and might act like a sort of dam, to hold the acrylic in place, even if it does not stick very well. Water could seep through, though.
If your sump is going to have several levels of water such as different chambers where the water might be pretty close to the top on one side, then almost empty on the other, then you will need fairly thick sheets. Also, a wider span, or a taller tank used as the sump will require stronger materials.
If your sump is not going to have such extreme water levels, or is smaller, then thinner material will work.
You would probably get by with something under 1/4" if there is not a lot of pressure on it, and not too wide a span, but I would go thicker if there is going to be more pressure. MAYBE: 3/16" minimum, to 3/8" max?