Rear light fitting

Never seen a metal one. But there's no reason for the original to crack, it's just a cover. I doesn't hold the light on or anything, the number plate bracket does that. I'd recommend checking that the brackets and bolts are correct and straight then either replace the cover or repair and paint.

... sir. I put a little alloy plate underneath and secured with epoxy and 2BA nuts & bolts.

Fill the cracks with carbon fibre reinforced filler, sand and respray.

Originals are quite thin and often crack, I have just fitted a recently acquired one which is much thicker but does then cause a fitting issue as it stands off from the back of the light bracket and pushes the front hole too far forward to line up with the tapped bolt. I shortened the pillar nuts to make it fit ok. Hopefully it will be stronger and not crack but I can’t remember where I got it from.

Regards, Al.

Triumph used an aluminium alloy bracket for the same square Lucas lens which I've seen retro-fitted to Nortons too.

Do you have the correct internal threaded spacer bolt for the mudguard / lamp fixing ?

The cover is only secured with small screws. As long as the screws are not over-tightened and fitted with the plain washer and spring washer, the cover should not crack.  The screws only need to be nipped up to compress the spring washers,  First checking the underside of the cover to ensure the bolt and pillars nuts’ contact surfaces are level would be prudent.