Your title is confusing as a lot of professional grade systems mount into something known as a "rack". Generally these use 19" (or sometimes 8.5") wide cases that bolt firmly to a pair of rails in a larger case or cabinet. Height of such computer cases is measured in U (height unit), and a cabinet can measure anywhere from 1U to over 20U in height.

These 19" cabinets are quite expensive, but should be abundant on the second hand market as long as you can transport them ('cause they're big and heavy). They sell shelves for such racks onto which you can place a normal system.

Yeh I've got an idea floating round back of my mind to use something like this, once we move somewhere with more space.

Good thing about wood is it wont scratch the towers and the spacing is far enough apart to have them sit upright, may even get 3 to a shelf?

I also had the idea of a proper server rack as well as I have a few rack mount servers and installing a few shelves like yawetaG said. Problem is they take up even more room!

For the moment they sit under the table or on top of each other.