In the middle of remodeling my sump right now also, but it is glass on glass and I didn't think it would work with glass and plastic.
Should have asked question here first LOL. Would have saved the " Why do you always have to modify things " Question from my wife...............

glue
 

try gorilla glue-this stuff expands so make a couple of dams with masking tape(the green plastic type)

still better than "I told you so" from the wife

This is what I've seen before. Weldon and nylon bolts.

http://www.thereeftank.com/forums/sh...3&postcount=10

http://www.reefcentral.com/forums/sh...25&postcount=6

Wouldn't you know it. I went and had a piece of glass cut to fit. Came home to replace the plexiglass so I had to cut off last of the black plastic brace. One end wouldn't come apart and the other end only released after applying a lot of pressure. So I finished the trimming, added more epoxy, clamped it together. Went to it this morning and it is bonded. I have the glass as a backup if it fails, but maybe I got lucky and have solved this problem.

As for the two links, I was kinda wondering what fastening device would work in this environment. Nylon screws would be perfect.
Thanks for all the good advise and ideas, If there are any changes I will report them...