My first case was a 84HP in a 19"-rack as well. It didn't take long to fill it, as was to be expected, but we all have to go down this road as it seems ...
I built my second case (6U 104HP) out of beech wood and bought rails separatedly. I use a TipTop uZeus as power supply. I had a lot of fun building the case, it was not difficult at all and came out significantly cheaper (and nicer ...) as the Mantis. We all know modular is not cheap, but the prices for the cases are far beyond reasonable IMHO.