Welded fabrications from tubes are awesome. I like bicycles, but lots of other stuff gets made this way. Make your own awesome roll cage for a race car, or a motorcycle chassis, or other stuff that’s not a vehicle, I guess.
I’ve used this website a ton to make tube mitering templates. Put your sizes in the calculator, print out a pdf, cut out the shape with scissors and trace it onto your tube with a marker. it works great. With some practice, you can hacksaw and file a tube miter into shape in under 5 minutes. It’s not as fast as a hole saw, but it’s much less expensive.