[Delphinus]

Well there are far more experienced acrylic workers here than me but here's my stab at it:

1. what thickness of acrylic should i go for?

I think if I was to build a sump I'd look at 6mm (which is, I think, the same as saying 1/4"). Maybe one size up from that, depending on cost and how big I intended the volume, but I think 6mm is a good starting point..

2. what kind of adhesive should i use?

weld-on .. there are several different numbers of it, and it all depends on how runny it is. The bigger the number the more viscous. If you take something like weldon #3 (which is like really thin water), you hold the edges together, apply the weldon with a syringe and capillary action pulls the weldon into the bond. Hold the acrylic in place for a few minutes, the weldon melts the acyrlic together, and as long as you have no air bubbles in your bond the end result will be solid. Maybe a quick squirt again so that the edges are sealed. Then let it cure for a day or so and then you're done.

3. what would be the easiest way to insure 90degree angles?

Basically with good cuts and edge preparation. Stuff sands like a breeze, don't worry it's really easy to prepare the edges.


...

It's perhaps a bit easier to see it in person than to try to imagine it. But it's really not all that hard espescially once you've done it a few times.