First, had hubby measure and cut the 4 peices, and I siliconed 3 peices together like this:

Then I put silicone on the old frame surface on all 3 sides, and slid my new framing over that...pushing into place:

Finally, added a couple of weights to hold everything in place while the silicone dries:

Add the 4th peice after you've slid everything into place. If you do this on foil, it won't stick to your working surface, and peels right off in case you had any excess silicone.
I'm doing the top of my 20 gallon as well, since that frame was really trashed. I had to repair a corner peice of glass that broke off while I was removing the old frame, which you can see in the first picture. Once I get the top frame on, only half of that will show. I'll put the filter on the other side to keep weight off of it.






