[jocko]

All aux tank setups are non-factory except for the NL2 option that only has a single filler neck with the on the upper part of the bed, somewhat aligned with the cab filler neck cap (pic 1)
Aux tanks are nice to have if you do any amount of distance driving. Personally, I prefer the aux tanks that have the filler necks inside the wheel well that do not require a door or bed cutting (barely visible in front of the left rear tire in pic 2)

I like the idea I think you're describing - it's one of those nice custom touches that look like they coulda/shoulda been factory and, at the very least, look much better than the aluminum flapper doors (pic3). You just have to go scrounge around a junkyard and find a door that looks good and chop out a donor section. I've seen them done with round doors also. I think I prefer the rectangle ones as they seem more in sync with the bed lines and they kinda put a modern spin on the aluminum rectangle doors. If I'm reading your post correctly, your current tanks are good but a prev owner just kinda hacked the bodywork when he hid the door openings - so this might actually be a great option for you to restore tank function and fix the bodywork of the po.

Having said that, the aluminum doors are definitely period correct and are the type of doors used on almost all dealer-installed or aftermarket aux tank setups.