Discussed this recently. The standard mud shields are as you say badly designed, really badly. They are there to protect the heater, clutch, brake etc assembles but don't protect the bottom of the bulkhead. I use Waxyol under body seal. It's an area that gets battered with crud and I redo the treatment regularly having recently replaced the footwells. It's a design flaw that could be fixed if you fabricated an extension plate. It could easily be fixed onto the floor screws. Surprised nobody has come up with a fixing.