Rotisserie in any vertical smoker is a challenge. Thick walls, like on an electric MES, makes it even harder.
Ideally you center your meat on a rod, secure the pointed supports, then easily place the whole thing, in a single horizontal motion, into slots for final balance and rotation...works great on a wide gas grill or
weber kettle.
But with a vertical, unless you want to cut slots on both sides from the front to the center, you're dealing with holes so there's no single horizontal entry path. Hence you you can't tighten down the meat supports since you have to bring it in at an angle with meat slid to the extreme end (if it had a handle, you needed to remove it by now and shorten that rod as much as possible) feed it through your "axle end" (requiring the hole be large enough to accommodate this insertion angle), then once it's in that
one side, you slide the meat to the center while you feed the drive end of the rod through the OTHER side hole (needn't be as oversized) on the motor side. THEN you have to secure the meat supports and tighten any bushings that keep the rod in place during rotation.
If you have any rub or sauce on your meat, this process is even more slippery and messy. And it's harder to do in gloves. Also don't try this with the cooker at temp, unless you're well-practiced.
Unlike most grill cooks, smokers tend to use wired probes (even if they plug into a "wireless" base station) to watch internal temps...uh, that doesn't work with a rotisserie. So you'll end up using the old curves showing time to cook per pound of meat for various oven temps. Err conservatively. It's easier to add more heat than to take it away.
There are one or two fully-wireless temp probes that I'd love to try but they cost more than the rotisserie. I suspect you'll need to tie them on to keep from falling out.
So it's doable but difficult to add a rotisserie to a smoker. The exception would be a pellet grill, unfortunately I bought a vertical pellet smoker so I have the same problems as a MES.
So I've personally found it much easier to add smoke to my gas grill with a rotisserie than it was to add the rotisserie to a vertical smoker.