I've made a spotlight for video recording out stuff I happened to have lying around (it looks jankier than you can imagine) but I'm stuck on the final part which is the means to attach it to a light stand. I made a bracket (of a sort) out of some bent strips of metal and I need to attach this bracket to a short piece of aluminium pole but I can't figure out how. The aluminium pole is 20 mm diameter and I don't have a 20 mm diameter metal drill bit. I'm afraid that soldering it (if that's an option for steel with aluminium) would be too weak and this thing is heavy and would cause some damage to whatever it fell on.

Assuming you want to attach the sheet flat to the cut off end to make a sort of "T" shape then a nut epoxied in to the end of the steel tube, and a set screw with a couple of flat washers to hold the ally on. Or the real way, knock a bit of wood into the end of the tube as far as possible, and use a wood screw to attach the ally again with a couple of flat washers.
Best , if you have access to welding gear, would be to weld a bit of steel flat on the end of the tube and bolt the aluminium on with another bit of flat on the top.

drill a hole in the flat and use a expanding bung or even a raw bolt it the tube, but don't over tighten it

Twist the middle section of the bracket 90degrees, flatten the end of the tube and stick a bolt through the lot.

Rough, but quick and easy.

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