They all work about equally well in my experience but I don’t use them on a day to day basis (or even own one) so take anything I recommend with a grain of salt. Even the General Machinery/Harbor Freight ones will basically work if you improve the clamping situation, and yes, 3d printed shims could work great for that. Cutting up some cheap 4130 to make shims works too.
I’ve used this one and it was fine:
https://www.trick-tools.com/Joint_Jigger_Hole_Saw_Tube_Notcher_RTN_100_3876
Yes, the tool’s not super stiff, and yes, you’ll run into more problems than with a full mill type setup. That said, it’s a ~$100 tool that will cut non heat treated tubing pretty well if you spend a little time modifying and learning the quirks. I wouldn’t run out and buy one to run a professional shop, but it’s a great alternative to having to file all your miters.
-Walt