But why would you ever put the plate at 0° when you can put it at 45°. Yes you need some height, but is that so onerous?
I am not saying they have to build something equivalent to the SLS/SST/Satun/Soyuz/Proton concrete pad infrastructure. Just put a 45°. Wedge, cone, pyramid, whatever shape works. Make everything out of steel if it works and is cheaper.
They don’t need to test with a spent F9. They already did with raptor engines blasting a steel plate in mcgregor.