Performance Products has a rubber door stop for the 356, but I've got to agree with the piece of a seat belt for our cars. The doors just aren't that heavy, and webbing works like a champ. I could make a couple leather straps for you, if you like, but the seat belt idea is a really good one.
If you go that route, an easy way to cut the seat belt is to heat a knife up on your stove, put an oven mitt on your cutting hand and apply the hot knife in a slow and deliberate stroke across the nylon. It seams the edge and prevents having to sew the strap when you're done. Same for the screw-hole; use a hot nail or ice pick.
Just make sure you've got the webbing stretched tight when you cut it.
My car (CMC) came with vinyl straps with inset grommets so you could screw them right into the fiberglass of the doors and body, and were placed a few inches up from the bottom. I never liked those.
I'll be using pieces of an old RCI lap belt this time, since they're no good to me after two years' use.
If you want/need something fabbed, lemme know.
In the pictures, you can see the hole in the fiberglass (first photo) where the strap was. It's the one with the round rust scrapings around it. Putting the strap between the hinges probably was intended to keep the door fiberglass from becoming fatigued over time -- maybe. Speculating.
The second photo shows the placement in the door skin, above the first screw from the bottom in the leading edge and splitting the difference to the second screw up. You might or might not have corresponding holes in your car, but my CMC had the straps installed when I bought it. Sorry the photo is so blurry, but I took it as we began my hack-job (many moons ago).