Might be worth fitting a nylon block such as this Peter.

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Trimmed to fit tightly into inverted U shape of gearbox cross member(after removing underseal and adding thin coat of protecting paint) it's sized to sit just 3mm proud of the metal edge and with a taper leaving more clearance than the thick aftermarket chassis protectors available. It's held in place by a couple of 6mm bolts deeply countersunk into nylon, through cross member and secured with nylock nuts.

Inadvertantly already tested coming off a road hump a little too quickly, there was just a dull thud as the nylon slipped over rather than the deafening crunch of metal hitting concrete. The tight fit ensures the metal shape is supported as well and shock spread across width of nylon.

Hard 66 nylon block bought here.