My mid-tower case, an NZXT S340, has four largish rubber feet on the underside. These provide just a tiny bit of clearance when the case is placed in my carpeted study.
However, as the PSU is mounted fan side down, I was a bit nervous about this lack of clearance. I then searched for solutions and some were quite expensive, whilst others were inelegant. All I needed to do was provide a hard platform for the PC case so that I got the maximum clearance for air circulation to the bottom of the case.
I found the perfect solution for my situation: a length of laminate flooring that I had lying around in the shed. I cut it to 600mm in length and sanded down the cut edges. The width was already 200mm, which is exactly the width of my case.
This allows me to place the case on top of the length of laminate flooring and then use that to slide it under my desk - no more heaving the case around under my desk. It looks good and provides good clearance for the underside of the case. Zero cost too.