Going back 15 or 20 years ago the wife and myself checking out a local caravan - camping show in Melbourne Aust there was a guy selling small basic spits that run on heat beads.
This was the time we were doing a fair amount of Car Racing and finding time to cook something between races alway seamed to be a pain ( there is always something to do to the car between runs )
and the beads being an open fire in the pits would more than likely frowned on.
Gas barbies were ok ( go figure ) any way this is what I made up as the mark one version that is still in use today now for camp trips as the racing car has retired
I started with a piece of 2.1 mm floor plate that was under my folder just the right size about 2 foot long
a couple of folds and some welded in end plates that was the bottom tray made
then with some 20 x 20 x 1.6 square tube made a couple of end "A" frames that side on using some short pieces of 1 " angle
the tops had grooves for the spit shaft and a mounting for a battery powered spit motor

then a few more panels folded up that slip together
and a home made burner
all was good

I also made up a clip on rail so a baking dish sat up in there - it work real well to

I remember the first time we used it - it was a sprint meeting at Sale ( RAAF ) Air Base
while my mates were chowing down on sausages in bread we were having spit roast chicken with roast Potato , pumpkin and onion with peas and carrots from a can that was punctured and sat in the bottom of the spit to heat
this unit has seen many roasts in its time and the burner has been replaced once after the first one rusted out
I didn't bother taking pics of the motor or stainless spit shaft you guys have seen them all before
the unit is 61 cm ( 2 foot ) long and 50 cm high and the temp gauge is a later add on
the rear good thing with this unit is if folds down and takes up little room and assembles quickly

all the panels just slip together and hook up a propane bottle and the spit is ready to go - the motor is just a small battery powered unit and I made the spit shaft from some 12 mm stainless shaft and of cause made up stainless fitting to suit