On my current pack, I was able to glue the seed trays directly to the frame with E6000.
BTW, I got one of those exact frames this weekend and went totally crazy hacking it up. I even went so far as to remove almost two inches of width from both the top and bottom bars to best fit the seed trays. Mind you, I'm working on a different backup, not Sandtrooper. Details to come.
Also, use wooden dowels, affix them with some hot glue and then use copious amounts of Gorilla Glue to secure the bond. It will expand in the frame and make the re-connections solid.