We'll assume you're using the standard small aluminum miter box available at most hobby shops, or arts and crafts stores. First go ahead and make the basic cut in the miter where you want the plastic wacked off. You'll probably notice that the tubing or strip is rarely cut perfectly straight in the miter. But, the outside edge of the miter box is perfectly square, so push the tubing to the end of the box (just a tiny bit past the edge) and use a sanding stick (block) to gently true it up. Spinning the tubing in the box while you're sanding the end will even help more getting it flat and true. For strip, you can also rotate it in the box while sanding to help assure a straight square end.
You'll probably eventually deform the edge of your aluminum miter box after many many sandings like this but it's worth it when you must have a perfect square cut. After a few times doing this you get the feel for putting most of the sanding force on the plastic and not on the aluminum miter itself. Anyway your miter box has four square corners to work with so it'll last a long, long time before getting a little out of kilter. Then go buy a new one.
