Since it appears your uncle will need this on a continuing basis, he may want to invest in a good copy stand. Then, he can simply mount his camera on the copy stand and be reasonably assured that it is plumb and level.
Cheap copy stands are cheap and somewhat inaccurate. Good copy stands tend to be pricey. Since your uncle sounds handy, he may be interested in crafting his own.
There are plans around the Internet for cobbling something together from scratch, but old film enlargers are dirt cheap these days, and can easily be re-purposed as copy stands.
Here is one crafted out of a Besseler 23C II, a classic, well-built enlarger. I picked it up at a camera swap meet. I had talked to the seller a few times during the day, but didn't bite at $10. At the end of the day, every one was packing up, and he ran up to me and said I could have it for free, if I also took another old enlarger for free that he hadn't been able to sell! How could I pass that up! (I attached the lights and polarizers when I got it home.)
It's worth looking for a stand (or enlarger) with a tilted column. This means your uncle can work on models of various sizes, without the base of the stand getting in the way on larger models.
Fast forward through an emergency medical move in which I had to leave that behind — I find myself with an 18" cube of my father's photos to scan and deliver to my siblings, after his passing. So, I went onto CraigsList, and found a Besseler 23C II again!
It is quite easy to modify, requiring only removal of a "C" ring to get the enlarging head off of a 1 cm (3/8") rod. Rather than fabricate the odd-angled aluminum box shown in the photo of my earlier stand, I plan to make a simple wooden platform, attached to the original head-mounting rod, then use a simple screw as a plumb adjustment. This is the way the original enlarger head was adjusted, so be sure to understand that before taking it apart.
With that in place, one can then use the "mirror" technique outlined above to align it… once! Then, your uncle can quickly and accurately shoot models as fast as he can put them under the copy stand, with no further adjustments needed.
That will deal with the "plumb" part; what about the level?
If you go to this much trouble, I'd rather permanently affix a quick-release on the copy stand, so you won't have to futz with the unpredictable "roll" alignment problem of attaching the camera via the typical 1/4" tripod screw. Then, your uncle is truly ready for "snap in and shoot" experience.
I use the Manfrotto 394 quick release system used on their 310 Mini-Gear Head. It is sturdy and stable, and rated for 10kg (22 pounds). It has multiple holes — if you mount it with two screws, the "roll" alignment problem goes away. Newer versions than that pictured actually have tiny bubble levels built in. But almost anything of reasonable quality will do, like the very popular Arca-Swiss system.