If I had a greenhouse I would buy a bunch of those thick rubber tile puzzle pads. I have seen them at Menards. The ones I saw were black which would be good for heat and they interlock so once you put them down they would not move around
That would be a good idea down the middle of the greenhouse.
I need gravel for drainage under the catwalk-like metal grate salvaged shelving because when I water in there - I WATER, LOL!
P.S. Last crop of sprouts were delish! I have lettuce seedlings coming up inside and out. :)