I often solder things to other things -- well we all do, I suppose, but sometimes these things need a bit of pressure to stay together while being soldered, and the traditional third hand, which holds your work while you hold the soldering iron and solder, doesn't do that very well. I remembered a few years ago when I took a flame-working class at the Crucible, learning how to work with molten glass. In that technique, the torch was fixed (pointing away from me) and I used both hands to hold the glass rods I was working on. I realized I wanted a fixed soldering iron, and to be able to use both hands to hold the work.
Turns out it was trivially easy to make a soldering iron holder out of a couple pieces of scrap wood, some screws, and a "spring grip" -- one of those spring grabby things you can push a broom handle into to hold it in place against a wall. Tools: saw, screwdriver, drill.
Step 1: Cut the Wood
You need one piece of wood in a triangle shape. I also wanted a base, so I could clamp the holder to the table, but you could bolt it down permanently if you prefer. I used a bit of 2x4 and some 1/4" plywood scrap.
You do want to assemble the finished tool so it holds your soldering iron at a good working angle, but doesn't obstruct the cord of it. I found that 45 degrees was too much, and used an angle closer to 30 degrees. The base only needs to be big enough to screw the triangle to it, and have some stick out for the clamp to attach to.
Step 2: Assemble
Now simply screw all three pieces together. Two screws to hold the base to the triangle, and two screws to hold the spring grip on. I got a little fancy and countersunk the base screws so it wouldn't wobble. And by countersink, I mean I got a big drill bit and just touched it to the drill holes I'd made for the screws. If you have a powerful screwdriver you can probably just screw them in far enough without bothering to drill out space for the head.
Step 3: Use
Do clamp or bolt it down when in use. The whole point of this is to make the soldering iron immovable! I'm using a clamp because my workspace is not dedicated to soldering but if you have room, attaching it to your table would be great.
Here's me soldering, with the iron held fast by my lovely new tool. It worked great!
As suggested in the comments by jiovine, it may be even better to use the opposite orientation, where the iron is pointed down and towards you. I tried this orientation in a dry run and I think I will probably use it that way instead of with the iron tip pointed up in future. (The hot bit is closer to Me that way, so a bit more care will be required.) It's a little tight because I didn't design my holder with this orientation in mind, so if I have to use it for larger pieces I'll probably have to modify the holder. But for small work it's just fine.