I'd like to make a tray for my battery, but future proof it against the chances of my next battery having slightly different external dimensions. Is there a standard that applies to the external dimensions of car batteries that I can work from?

I would say that like for like , for example an 063 , even from different manufacturers would have the same dimensions.

I would say that like for like , for example an 063 , even from different manufacturers would have the same dimensions.

You would need to add an adjustable lower clamp as found on most trays or what I did was to use 2 wire hooks with threaded top and bitterly nuts with a V shaped top bar, I think you can buy these from CBS
You just need a wedge for different width batteries

Google is your friend.

https://www.puretyre.co.uk/car-battery-specification-chart/

theres over 50 'standard' car battery dimensions, also the shape and layout of the terminal posts vary

https://www.puretyre.co.uk/car-battery-specification-chart/

but as above all 063 should be very similar dimensions

Get the battery tray off a Rover 75 it will bolt straight to any flat surface. Also comes with a plastic battery box with lid.