I’m looking for either a handle that can be screwed onto a thru-axle or a dummy thru axle with a handle to make it simpler to hold a wheel and spin it at different angles to distribute sealant all over the inside of a tire. My thru axles are 12mm x 1.5.
Does anyone make such a thing?
I mean I don’t know that I‘d get enough usage to justify buying this, but Park Tools sells this: WH-1 Multi-Position Wheel Holder - which does exactly what you want. It’s $140 bucks though. (If I were to try to build something like this for myself, I’d definitely make the dummy axle removable because in my workshop, I’d constantly injure myself on a fixed axle sticking out of the bench).
I use the Park Tool WH-1 for getting the sealant spread across the interior of the tire. Allows for verticle/horizonal/45 degree positioning that you can spin and spread the sealant. Supports 12 and 15 millimeter thru axle as well and it’s easy to create one for quick release. Also it great for installing cassettes, adding seallant and installing tires as well as doing general wheel maintenance. You just need a bench vise to clamp it in, but there are other ways to use it. Probably work just fine in your hand as well.
I’ve wanted a WH-1 for a while, but right now I don’t have a place to use it. I did find a file for a 3D printed (wheelbarrow) handle. I think I can modify that so that it takes an inset M12 x 1.5 nut.
Everyone needs more 3D printing in their life, right?
I use a 10mm hex 
I bought one of these Park wheel holders a couple of months ago and as it turns out I use it once or twice a week for the wheels of my four bikes. It was a tough purchase as I consider it to be expensive but given I use it often it has been worth it.