Its called "Weld On"
comes in different grades that have to do with how thick a liquid it is.
I use Weld On #16 since my gaps aren't perfect.
Weld On #4 is thinner and used with perfect gaps.
Stuff is not a glue, but a solvent cement. Actually melts and refuses
the acrylic at a molecular level. Like a true "weld" with metal.
Leaks are non-issue. This solvent makes really nice seals, and on top
of that I run a small bead of silicone on the main shell's inside corners just in case.
You just use off the shelf pieces of acrylic from lowes. Tape them together
loosely and add cement. The cement weeps into seems and sets up in couple of hours.
Google: Working with acrylic. Lots of DIY artcls out there from the reef guys.
Excellent one
http://www.advancedaquarist.com/2007/7/diy