I came up with a creative way to mount my spare trolling motor. To prevent the tubing from crushing when the bolts were tightened, I inserted 3/4 OD aluminum rod into the hollow tubing, then drilled through it for the mounting bolts. Appears to work just fine. I have not had it in the water yet, but I'm sure getting that fishing fever!!!!!!!!
(For peace of mind while driving down the road, I replaced the OEM set screws with 1/4 inch bolts directly through the tubing.)