Squaring the ends with a file is certainly possible but it's a brutally slow and skill loaded solution. The lathe is far quicker and better.
A tubing cutter will leave a bad ridge as mentioned already and also an edge that is far from flat. And then there's the squish it makes on the diameter to make the inside smaller in diameter.
If you don't have the tools your best bet is to stick to buying them. And why the insistance on only one? Granted it would look neater but not if it's raw pipe cut with a tubing cutter.