I like cross dowels. They’re those little metal dealies that are used in a lot of furniture and cabinets from Ikea and places like that. You can buy them ready made, but I have some useful equipment to make them.
Here’s a nifty drill fixture for drilling the hole through the aluminum bar.

This Swiss made Tapmatic is as dependable as a Swiss watch and makes short work of tapping dozens or hundreds of holes. It’s tapped tens of thousands of holes already.  One of my favorite tools.

Threads cut in about the time it takes to say “threaded”.

The last step is to cut off the end of the rod using the “cold saw”, which is a little like a carpenter’s cutoff saw. The blade moves much slower, though, and a pump circulates a water/oil coolant mix.

Several machines were set up at once, and there was some walking between them, in flagrant violation of “lean manufacturing” principles. Oh well, I’m not ISO 9001 certified either, but I do good work for a lot less than the big guys.

These are half inch diameter, made of 6061 “aircraft” aluminum, an excellent alloy for general purpose applications. They could definitely be supersized to three quarter inch diameter and used to hold framing lumber (2×4, 2×6, etc.) together.