Seat belts as tie downs

Seat belts are too slippery
Cam straps are inexpensive and work well. Get thickly-woven ones with good buckles. Some of the straps I’ve seen and used are noticeably stretchier and thinner than the better ones.

My favorite is Salamander/New River Gear poly straps with a double-layer of fabric padding under the buckle. Available in many lengths, at least a couple of widths, different colors, padded or not, etc.