Here is the new indoor "Go Crate" I made for my son:

Here is how to build a Go Crate:

To build one I took these casters and attached them to the bottom of a milk crate. You can skimp by getting casters without brakes if you want. Lowes and Home Depot do not regularly stock spindle casters, but if they do, you can get a light duty set, mine were left over from my old table saw. You can get a milk crate, um, from around...or here. The crate that works the best is one that has smaller holes in the bottom. To make sure the go crate is steady, put a washer on the spindle, then insert the spindle into the furthest corner hole, then put another washer on, and finally the nut. After that I cut a piece of cherry plywood and fit it into the bottom of the crate perfectly. In fact, it is so tight that I had to stand on it to force it into the bottom of the crate. After that you can add a pull string or just leave it as it is. Because the casters are all four-direction casters, the Go Crate tends to drift on a string. The casters are very, very heavy and as a result the Go Crate is very unlikely to tip over, even with a kid in it. That said, use your common sense when putting a kid in one.