[Shop-talk] Aligning round tubing for mitering?

[Obaa]

I'm trying to build a small rack/platform using some 5/8" round tubing.
I'm planning for it to be rectangular, with the tubes cut at a 45 degree angle (don't have a tubing bender).  I'm going to notch 3 crossbars with a 5/8" drill, to form a nice, flat platform.
The problem is, I don't know how to align the tubing to keep all the cuts square with each other.  Is there a simple way of doing this?

Thanks!