I made this about two years ago to hold my long lens and camera in place on my tripod. (For night time & wildlife photography)

It's made from slotted aluminum extrusion (that was cut in half to align the lens to camera body) and a piece of 1.25" square aluminum tubing.

Its bolted together but I might weld it in the future. There are 8x 1/4"-20 tapped holes running up the top rail so that it can be adjusted on the tripod for balance. There is a 1/4"-20 thumb screw in a slot for adjustments on the bottom rail, coupled with a ground down tee nut without the barbs so that it doesn't fall out, to hold the camera in place. Another 1/4"-20 screw with a few washers is in the lens' tripod mount (its only ever used with this setup anyways but it can be removed) that slides down the inside of the track on the top rail so that the lens can be extended.

With this setup I can get full frame shots of the moon or a basket ball about 1/2 mile away on my t3i at 1300mm.