Ah....you want me to give up all my secrets... 
I really don't use glue at all. I used black Silicone Sealant/Adhesive lightly placed on the inside of the shell. I Like using Silicone because it is somewhat non-permanent in case you need to replace a bulb and black so the light beam does not back light into the shell. I might also add that, where possible, I make tubes for the bulbs to slide into so the silicone need only be added the the rear of the tube.
I really don't use glue at all. I used black Silicone Sealant/Adhesive lightly placed on the inside of the shell. I Like using Silicone because it is somewhat non-permanent in case you need to replace a bulb and black so the light beam does not back light into the shell. I might also add that, where possible, I make tubes for the bulbs to slide into so the silicone need only be added the the rear of the tube.
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