Just some thoughts of mine from looking at your pics that I would probably do if I had what you had to work with.
I would mount my propane tanks on the elevated portions of the bumber on each side of the door. The diameter of the 100 pound tanks is just under 15″ if I remember right from last week. I’m not sure how far your bumber comes out but if it doesn’t come out that far then you could just get some peices of of like 1/4″ steel plate and fasten that to your bumber then put the tanks on the plate. On the side walls of your truck then you could get some 1″ angle iron about 36″ long and attach those pieces on each side of the tank and then make straps the same way I did.
The flat bar is SUPER easy to form and yes I just layed the tank down, put the flat against the tank and pushed down. Minimal effort.
If you go alumimum you angle you’ll need structural alluminum. The steel they sell at Menards though is not some crazy industrial stuff. It’s strong but not super thick so if you have a hack saw you can cut it that way or a saws all or a angle grinder with cutting wheel.(thats what I used)
For the platform, what I did is used a spray on rubber sealant. After dried it is VERY durable. This way I don’t get the metal to metal contact and scraping then rusting. Its nasty stuff when wet but makes a very nice barrier. They sell this at Menards as well. You could also get rubber sheet like 3/16 of an inch or so to place on the bottom if you want.
Hope I helped some.