With Door Cars and the over sized window net required, or shown in the rule book to keep your legs / feet inside. I have seen some cars with Sheet Metal covering the X brace in the door area. If it is fastened to the inside of the X bars would that be acceptable for the lower portion ?
As a safety tech inspector I would say yes, having sheetmetal fastened to the inside of the X bars would be acceptable for the lower portion. Two things:
1) Watch out for sharp edges
2) Don't forget the upper portion (window net)
While you're trying to figure out your limb restraint setup consider that the doors are typically one of the first things to let go in a roll over. Imagine that there are no doors on the car and that you want to keep your legs/feet inside at all times. If you go about it that way I think you'll have a good idea of what you need to do.
Best of luck.