What's the best security for one of these? Another "mother" project, hers is
an old Garador one with the release lever in the middle of the door but
she's lost the keys so it's permanently unlocked.

I looked at the "Garage Defender" ones but they appear to have mixed
reviews. And fitting needs to be simple, basically she has a large, level
one-piece block of concrete of which half is the driveway and the other half
is what the garage is built on. I don't mind having a go at drilling the
concrete for the fittings but chipping out rebates really isn't on my
wishlist.