When assembling make sure you do not stress the rear mudguard at all, especially the two front mounting point points where the mudguard fits to the RFM. Don't tighten the nut and bolt to pull the mudguard as it will stress it so its best to fit a spacer or shim to fill the gap there.
After nearly 30 years of ownership of several Vin's, I have never had a cracked rear mudguard but I never use the rear stand if possible and I think some cracking of rear mudguards is from using the rear stand. Just look at where the stress of putting a bike on the rear stand are transfered through, basically through the mudguard. When you pull on the rear mudguard lifting handle which pulls the bike backwards that stress is transfered through the mudguard mainly via the two forward mudguard fixing points which are attached to the RFM. When you are pulling the lifting handle backwards it tries to rotate the mudguard arount the lifting handle to RFM lower bolt.
Well thats my theory, I will now go and stand in the corner.
Simon