Steven Briggs wrote:

Don't be tempted to use hardboard, it will sag _a lot_ in garage
conditions (i.e. a bit damp).

12mm CDX (sheathing) ply would be a good choice.
Full WBP ply if you can afford it.

Of course I can afford it - I'm absolutely loaded. On the other hand, I am a
skinflint of the highest order!

Seriously, ta for the advice - genuinely appeciated.

Rick