The only tubes that require a miter cut are the two outside exposed tubes! The center tube (motor mount tube) is under the two other larger tubes. Why bother cutting an angle in the front of it?
Wrap, pencil mark and cut with a sharp knife.
I’ve read where builders have had problems making these angled cuts. If you have a block with 220 grit sandpaper, you can always clean up a rough cut.
Look at the top of the tube and you can see the white tube edge pushed up and hanging on.
Roll this edge over with your finger and sand down to remove it.
Follow up with 400 grit.
I’d also recommend some thin CA around the edge to strengthen the tube ends. With the CA on the edge it’ll be stronger and smoother after a little light sanding.