At the risk of getting "the speech", I'm going to echo what a few others have said here: there is nothing wrong with the DF supplied strap. I can put my entire 200+ pound, pseudo sasquacth-esqe weight on the floor pan, and bounce up and down very comfortably with absolutely no feeling that I'm going to fall through the floor. Is it rock solid? Not really. Could it be beefed up for some obscure and extreme use case? Absolutely.
I will go out and say that adding any kind of support on the drivers side is 100% unnecessary. There is absolutely no way you can enter the vehicle and get yourself into position all while putting your entire body weight on the floor pan. The steering wheel position, body work, seat, etc all make it physically impossible to do so. The passengers side is a little different, but then again it's practically impossible and very unlikely that someone will need to stand up straight with their entire weight on the floor pan. I find that most of my passengers put their initial weight on the seat anyways.
Don't get me wrong, this thread has a lot of great information in it and clever solutions. When I first started my build, I wish I had gone this route. Now that I'm almost complete with mine, I'm glad I didn't as I would have found it to be a lot of unnecessary extra work, time, and money. Again, that's just me. To each builder, their own.