One day at an estate sale I picked up a universal rotisserie for a charcoal grill for $3, and decided with such a bargain I would figure out how to make it fit.
Getting it to work on this grill was pretty simple. First I cut a slot in a piece of 3/16th thick steel so that the end of the rotisserie rod would be able to pivot in it. The slots built into the grill housing were too thick, so I mounted this now-slotted piece of steel inside, using washers to provide the necessary clearance (photo 2).
I used a piece of right angle steel (photo 3) to mount the motor. I fastened the steel angle to the motor's face using two of the four screws on the housing, and onto the side shelf of the grill.
The rotisserie rod was a bit too thick to rotate in the slot built into the grill, so I ground off the four corners until it had sufficient clearance to rotate (photo 3).
That's really all there was to it! As best I can tell I saved about $86 dollars (not counting the shipping).