These fit over the Keychron K4's existing legs and add an extra ~16mm to their length. The fit is pretty snug but it doesn't take too much effort to push them on.

I haven't tested on other Keychron boards but I'd imagine these will fit other K-series (non-pro) boards at least.

The hole is 25.25mm by 7mm, which is based on rough measurements of the legs using calipers. Width was fine on the first version, but I increased the height from 6.6mm to 7 since first fit was a bit too tight. Depth of the hole is 16mm and length of the entire piece is 32mm.