I always used paper,until I saw that Bill Horan uses electrical tape. That's what I use now, though I also sculpt straps from epoxy putty when I need more control over shape. I never use putty for rifle straps though.
I've done the masking tape method, but I don't like the texture, especially in smaller scales.
I make buckles from steel or copper wire. It's a little tidious, but works for me. I hold the wire flat on the table with an X-acto knife blade and bend along the back edge. Then I move to the next point where I want a bend and do it again. Then I make the final bend to close the square. The excess is then snipped off with my sprue cutter.
You can use the varying thickness of No 11 blades (as in the width from back edge to cutting edge, going from the point down to the handle) to help create the appropriate size of the buckle. Just rest the cutting edge against the first bend and then slide the blade until the back edge lines up to the desired spot for the next bend. Just use a length of wire long enough to hold securely in your fingers while you work.