When I build my sump, I used a combination of weldon #3 (similar to 4 but doesn't dry as fast) and weldon #16. I used weldon #3 using the 'pins' method outlines here
http://www.melevsreef.com/acrylics/s...chvy_sump.html
and weldon 16 to fill in any gaps.
Since the tank already exists you could probably get away with just using weldon 16 to install the baffles (I found some at Industrial Plastics and Paints).
HTH