Hello OP, PMed you my intentions to order a 6.5 inch greenie. Going to stick it in one of these
I thought about taking 2 of the 5.5 inch ones and putting them in this tube but I'm too cheap for that, I'll just dremel the tube down to size. Still if anyone else wants some ideas on how to mount these, this ebay item would go great with two of those 5.5 inchers.
Found the rubber stoppers I will use... "00" size should be perfect. They will fit inside easily on one side but be snug on the other for a nice tight fit. Cheap too. Just drill a small hole in the middle to seat the tritium, or you can find them with pre-drilled holes.