. I now have 6 builds waiting for me to figure out how to make seat belts for them (or get someone to let me use their credit card).
Seat belts can be very easy to make. I've done them a few different ways.
These seat belts are just ribbon with buckles made from Evergreen strip. Ribbon will shred and have frayed edges if you try to cut a standard piece. To avoid this, take the ribbon and coat both sides with white glue. Make sure it sinks in and doesn't sit on the surface. Then the ribbon will cut nicely. They also make ribbon in the right width, probably hard to find in the real world, but the aftermarket does sell it as seat belt material. Of course at a premium.
To make the buckle, I just took a length of Evergreen and round off the edges. Once I had the look, I cut the buckle off the stick. It's easier to work it with a longer length so don't cut it to size until you've got the shape. Spray with silver paint. Note that seat belts are more convincing if they're random like in my picture.
Here's another way to do seat belts... in this case, I took those hokey plastic seat belts that came in 1960s kits and cut off the buckles. Note that I would have been much better off if I had sanded them thinner first. Then I used the premade width ribbon to make the belts. They are way thick in this photo, but once in the interior you can only see them from above so they look a lot better.
I'm amazed that I didn't take interior photos of this one prior to it's final assembly. Anyway, another way to make seat belts is with masking tape. Take the tape and spray the sticky side with Testors Dullcote. That completely neutralizes the glue and now you have nice thin material to make seat belts. Just spray paint both sides what ever color you need, cut strips to the right width and go for it. This was the first model I did shoulder belts on, using this technique. You can see the belt hanging near the B pillar in the photo.
I've also used aftermarket photo etch seat belt kits. With those I don't like the buckles as I think they are way too thin (you could back them with some plastic to get scale thickness) but the male ends are really perfect! And work with any of the belt techniques I mentioned above. I bought a Detail Master photo etch seat belt kit for something like $5 and there were enough buckle parts in it to do several models. So it wasn't overly expensive. Still, I got the idea you were looking to do belts with available materials, thus the info I posted above. Hope this helps!