How to install internal overflow on established reef tank

[gregzz4]

Are you just wanting to 'hide' the plumbing ?
If so, you can use Orca underwater glue
Brace the boxes against the glass with some rocks or such and put the glue on the inside
Once it's cured the stuff is very hard to remove, but I don't guarantee it'll hold acrylic to glass longterm, so you may need another bead on the outside

If you're wanting to make functional corner boxes that won't leak and drain your whole tank, I'll back slowly away from this thread