Looking around for storage solutions after losing the backseat to forward facing child seats, I came across the ammo box thread
A few weeks of searching kijiji turned up this;
It's a WW2 Ammo box, labelled BSMS/c 1944 M152 I (interwebs
says it carried mines during the war).
With a few bolts through the back mounted to t-nuts slid in the track it sits secure, level, and the lid opens clear of the canopy;
It's light, tough, removable, and will carry First Aid gear/ recovery gear/ fire extinguisher. Plus it can slide on the tracks if I need to fit things around it.
It isn't shiny and new, but for 70 years old I can dig some patina.