I have noticed a few big holes in the boot floor. I see one with a plug in it. I have been searching for the plugs but cannot find them. A piece of tape will hold for a while but it would be nice to get some proper plugs.

 

Used to let the paint drain out when the body was dipped in primer and to locate the panel on the jig for welding.
You can get rubber plugs of all diameters on ebay. Blind Grommets they are called

 

The big home improvement stores carry a variety of rubber plugs as well....

 

The original plugs are steel with little 'fingers' that bend to lock in place.

I have 3 X308 sedans so I just 'borrowed' a few from the abandoned cars.
A salvage yard should have them in sedans or any older Jaguar,
Just bring a sharp-nose pliers to gently bend the tabs to free the plug.

I did measure them once but I forget the diameter.