You're not the only one who takes issue with non-opening doors on playing fields, trust me...

It's been a pet peeve of mine for years too, and I've been giving it a lot of thought over the years, and the solution I came up with is virtually the same as yours! I did think up a way of locking the doors in place when they're up, with a simple sliding bolt system made from metal wire. But when lasercutting, it would be very easy to cut out a suitable hole for a magnet to fit into, so no need for complicated (and prone to malfunction) mechanisms.
I would be very interested in such doors becoming commercially available! Single doors, wide (2square) doors and portculli as well.
And the same would go for scifi looking doors, both single and wide (but no portcullis required there

)
Very useful pieces of kit there, to be sure...
