Mount servers at the center?

There are servers with rack-mounting kits that specifically are designed for center mounting. Usually they have ears that mount to the rails at the center, then another set of ears that will mount to the front of the rails so you can screw the ears of the server into it, so they don't slide forward as you are pushing cables in the back.

However, the servers I've seen that have these rails were years ago on Supermicro servers with the sturdy ball-bearing rail kits. The servers in the last several years have all been missing this capability. So mostly you'll just need to check the capabilities of the servers.

Another option I've done in the past was to get 2 2-post kits and use them for the front and back rails making my own 4-post rack. I used a double layer of plywood to bolt the bases into, then I used a cabling ladder bolted to the top of both and then mounted to the wall to stabilize it, say for an earthquake. Worked very well and was effectively a 4 post rack.

If you don't need maximum density, front and rear mounted shelves and sit the server on those. But that won't be something you can screw into, and if it tips any it could all come tumbling down, but also you may not be able to get servers in the mounting space between the shelves, which could half the density.

Mounting a server in a 2post rack can be accomplished fairly easy in both a centermount or flushmount configuration. Depending on the manufacturer, most modern servers have alternate rail kits that are compatible with 2post racks.

There is a second option available that uses a conversion kit. This kit has (4) support brackets that allow you to use your 4post rail kits in a 2post rack. There is a critical dimensions that has to be determined before using this kit. The server chassis with the 4post rail kit installed cannot exceed the opening of the 2post rack, which is 450mm wide. These kits are available in multiple sizes ranging from 2U to 7U.

Another option available if the equipment exceeds the 450mm width limitation is to use a universal rail kit. This rail kit is a non-telescoping kit that will mount most equipment into most racks and cabinets.

It can certainly be done but and the kind of shelf Mark is talking about can easily be constructed or purchased but it still ands up with a server that is effectively balanced instead of properly mounted. From an engineering perspective you are placing loads where they don't belong and on parts that may not be designed to handle them safely.

So, Possible? - Yes. Advisable? - No. My opinion is that if you cannot currently afford a four post rack you should consider not installing the server in a rack just yet and wait till you can buy an appropriate rack. You only need one bracket to break and the resulting damage could be significant. Also consider that you cannot (safely) use rack slides when mounting in a two poster, because the leverage resulting from a server slid out for maintenance will almost certainly result in breakage and having to remove a server from the rack just to replace some component is a pain in the you know where.

I have mounted a server in a 2-post rack by getting a 2-post shelf.

The shelf was a 4U job designed to carry 50Kg. The server was a 2U server and sags at the back, but it's been in the rack for 18 months now and it hasn't broken yet.