This is just my two cents.
I've used a commercially built one and didn't like it. (It was a Kingfisher Cooker--not sure how prevelant they are). It was very easy to get the tray off balance when loading or unloading the meat. I dumped two pork butts out of a tray simply by lifting a third up to pull it out.
The other problem I experienced that day was you couldn't leave a temperature probe in the meat. You had to manually stop the movement, open the door, and check the temps. That's not a bad deal--unless there is a "nervous nellie" helping to cook. One guy was constantly probing the meat to see what temp it was at. It was hard to keep the heat regulated with that going on.
I'm sure others have better results than I've experienced. Maybe they will post with thoughts and possibly pics. Good Luck.