Before removing front piece from printer bed apply Shoe Goo or similar adhesive sparingly to letter fill pieces and 10 points around perimeter of gel first layer. Only small dots are needed. On perimeter dots keep glue back of letter openings. Put a small weight to apply slight pressure while it dries. I used a block of wood cut to fit into opening and a spray can for weight. After drying, insert 2nd gel and diffuser. Next a few dabs of hot melt glue to hold everything together. I used 2 gels to intensify the color. This is wired to power supply so a .5 amp fuse is recommended. I used 22 gauge wire and 2.2k for the resistors. Have fun.
3D Solutech Pla
I did this in draft mode. Nothing special to say. No supports needed as long as openings are up. I did print with 3 walls.