I am rebuilding my proton thrower after a Halloween misadventure. I am using ThrowingChicken’s great box. On a trip to Lowe’s (box in hand) I noticed that a 1 1/4" Shop Vac extension tube might make an ideal rear handle. It is the perfect color, texture and diameter and is made of a tough but easy to work plastic (ABS, maybe?). It tapers slightly and so this allows you to cut off the smaller end at just the point you need for it to fit tightly into the rear opening in the box, making the puttying of the seam very easy. The inside end can be cut (I used the band saw) to shape it so as to occupy as little room inside the box as possible, not block the grill slots, and still leave enough material to be bolted and glued to the inside of the box (I used epoxy and two nuts and bolts).
Save the cut off piece from the smaller end and you can use a portion of it to fasten inside the back end of the handle to reduce its inside diameter to nicely match the outside diameter of the wire loom, thereby making a sturdy, professional-looking camouflage for the connection between the loom and the handle.
Hope this helps! Sorry for the absence of inside photos but I am not taking exoray’s light kit back out just for you guys! 
Drop me a pm or an email: [email protected] with any questions.
Steve