Not really very difficult if you size the spit rod correctly. OneGrill spit rods are made with handles, counterweights and bushings that all come off quite easily and their universal motor mount is very straight forward. Placing a loaded spit rod in your cook chamber, will be a lot like loading my Santa Maria Ranch Grill, not very difficult for one person.
If it were me, I'd plan on a spit that is 3" longer than the OD of your cook chamber for the motor end and another 3"-4" for the handle, counterweight and bushing; call it cook chamber width plus at least 6", but not to long. The diameter of holes you will need to drill will be governed by the bushing diameter, I think they are about 1" to 1-1/4" so a 1-1/4 bimetal hole saw should do nicely.
To load, you will have the handle, counterweight and bushing off, insert the motor end first (but don't install the motor yet) and push it through the "motor hole" far enough to get the "handle end" through its hole. This means that whatever you are cooking needs to have the same 3"-4" clearance you allowed on the handle end of your spit rod. At this point your loaded spit rod is in the cook chamber just sitting in the holes you drilled. Attach the motor in its bracket. Install the bushing from the outside with it aligned in the hole with the bushing just running in the hole. Add the counterweight and handle and your in business.
The OneGrill site
http://www.onegrill.com/ has plenty of info on options for spits, bushings and motors. Personally, I would go with all stainless.
EDIT: I'm thinking left to right (instead of front to back) will work better.