It would be easy to make a metal lid with diamond plate and have it sunk into the concrete as well. Throw on a couple of gate latches with pushrods on the inside to open them and you are all set.
I should have read down the thread for the lid. Yours makes more sense than the original flimsy plastic ones.
If you don't have some sort of solid, lockable lid aren't these just launch tubes?
Russian fallout shelters often have a metal door like a manhole cover that is hinged to ROTATE to the side. That is, it does not open to the outside or the inside, it just rotates to the side. They have a flange on it so that a hydraulic car jack can force the door open against debris.
Clever designers, those Russkis.