Fit an immobiliser switch to disconnect the battery if you are not using the car for sometime. A pain in someways as you’ll need to reset the clock etc but in the absence of power or the inability to use solar panels I don’t see what other options you have.

I’ve akways been a sceptical about solar panels but last year we rigged up one...about A3 in size to the battery on our really did trickle charge it well!
I’m a convert.

1936 Austin Seven Ulster Replica