For a while what I've been doing with my tower is get a slab of ply or whatever wood slightly bigger than the footprint of the tower and get a set of plastic rounded furniture feet/sliders, which either screw in or are self-adhesive...
Stick those on the underside and not only will it keep the PC raised up a little further from the carpet, it'll also absorb some vibrations, and if you make sure to leave a moderate amount of slack on your cables you can easily slide the PC forwards to more easily access the rear I/O ports or take the side off to do work on it etc.