I had someone do it for me from a thick iron sheet,,, can't remember how much it was thick,,, 1.6mm IIRC,,,
anyway, aluminium is way too expensive as wells as steel,,, iron is heavy but it's very strong and cheap,,, you paint it and watch for rust every now and then and that's about it,,, [but bear in mind that here in roma we rarely have snow so no salt for us].
OMP did a sumpguard in aluminium for the mk3 beeza but that costed something like
280 euros back in 2004,,,