The floor of my apartment has quite a slope toward the centre of the room (I think it is something like 1/2″ over 4′). To keep the layout level I could use levelling feet as found in home improvement stores. However, having the layout on casters would be the cat’s meow, since I could easily move it about for cleaning and, more importantly, for operations. The picture below shows the design I came up with — I tend to over-build things, so I’m sure there is a more elegant way to do this, but it works just fine for me.
The 2″ diameter caster is bolted through a 1×3 poplar block (my entire layout is built of poplar). Through the centre of the block, I have a carriage bolt, which in the picture, has three nuts added to beef it up as well as for tightening. Above the nuts is a washer and the number of these take up the variation in height due to the floor. A standard t-nut for levelling feet is applied to the bottom leg and the assembly is threaded into it via the carriage bolt. As these nuts have shallow claws and can be easily pulled out, I made a retainer of 1/16″ aluminum with a .080″ styrene sheet spacer to firmly hold the t-nut in place with two button head screws.