I'm building a number of doors for the beach house, and I wanted to avoid the problem of not holding them solidly while I routed for the hinges. In the past, I've clamped them to my workbench or regular saw horses, but they still moved around a bit.
I looked online for door holders, and most of them were too complex. They have clamp assemblies or wedges at the bottom, which seems to be overkill and too specialized. Here is an example:
You can hold a door with that thing, but not much of anything else.
I decided to go a simpler route, making heavy duty T-shaped stands - door horses - and they work great.
I need to do a post on my "dado machine" - an industrial 1950's Dewalt radial arm saw with a dedicated dado blade on it. It makes it very easy and quick to cut slots and dados.
Theses are fairly heavy and hold the doors really well. I could have just as easily make them from 2x4's glued and screwed together. It would have taken three or four 2x4's - about $10.
I hold the doors with clamps at the top and that's plenty of support. If needed to, I could move them to the ends of the door and clamp the bottoms as well. I could also hold full sheets of plywood this way.
I thought about drilling holes so I could put dowels in to raise the door up, but I don't think it's needed. If I want the doors higher, I can cut a couple of blocks and clamp them to the posts.