It is surprising how well a rotten base can be patched, hammer out smallish patches to fix the worst bits, if nessisarry, tempoarily rivett in place or g cramps and then brase weld into place, that and hours of wire brush in a drill cleaning, then paint with generous ammounts of paint. Foams need not be perfect I'd think as the cover tends to give the shape and smooth things over. Obviously getting it exactly right will take more time / attempts to get just right (but if accurate enough joins or repiars in foam vanish once covered). Or just hang in there and wait for a k3 seat to pop up out of the woodwork. There may be 2 in my loft, but as 1 is a known mess and the other is not known whether it was a 500, 550f or k3 and they are mixed with several cb500 seats I am not in a hurry to dig one out - the remaining cb550k3 is in bits too so trial fitting isn't possible. I'd be trying to fix your original, it looks better than the last one I did - not even sure that was a honda, might have been for my mates yamaha xs250.
