What to Use to attach Plexi to Plastic????

[ensquire]

I have a cross brace on my tank that blocks out my light and I want to cut it and glue a piece of plexiglass in it's place. I have tried epoxy and it didn't bond well enough. What would be the best product to use ?
Must be somewhat safe as the brace is just above waterline in tank.
Would another clear product work better than plexiglass ??

Thanks