This is meant to support a 1U or possibly even a 2U server. I've designed it to use M6 nuts and bolts and you can use 2 or 3 bolts per support. In total you'll need 4 of these (2 of which are mirrored) to support the server from all 4 posts. I'm using two M6x30mm bolts and nuts per support and I'll probably see if it can support my 4U server later. Rack rails are certainly preferred but if you're on a budget and your server isn't super heavy then this should work fine.
I created rack ears for my Linksys network switch and then adapted it to this support with inspiration from this model: https://www.thingiverse.com/thing:3508516
The author remixed this model.