There's a neoprene based coating for koi pond concrete. Pond liner works too.
I'd avoid wood, black mold, sagging....
If sunk below the slab in a basement, one gains vertical space while letting the earth cool nutrients.
I put my water right to the bottom of the net pot. Used to do 1" below but noticed roots browned above the water line. As long as the base of the stem isn't staying wet you shouldn't have issues with damping off.
I'm lost. Biggest loss would be from inadequate runoff due to lack of a proper drainage system, thus creating a salt buildup in the medium.
You thought about grommeting the bottom edge of a 5 gallon bucket for a 1/2" line to floor drain or sump? Drill 1/4" holes in bottom of another bucket...
To be honest I'd say yes. Shit, ice cubes freeze in less time right? Pre freezing the second lot wouldn't hurt, but I'd still toss the packed tube in for a few hours since the material would likely warm during handling.