I just talked to a bloke from the RTA (Roads and Traffic Authority for you foreigners) who says I can go with the Brickboard suggestion to just bolt in a plastic tray. Normally they are very strict on insisting that anything cut out must be welded back (no riveting or gluing steel plate into rust holes), but in the case of the battery the only requirement is that it be secure.

There's a weight of my mind. I cut around the strut that holds the tray up, so the mounting points are still intact, so anything I find that's the right shape will just sit neatly in place. When I have the car blueslipped, they just want to see that the battery is secure, nothing else.

That's sorted all sorted now, thanks for your help.
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Drive it like you hate it