Hi Mike
How much slack are you talking about? and I presume you arnt talking about the the round rail? Linear bearings are adjusted by changing the ball size, but thats normally just to set the pre load. I've played with the thk ones and there not to bad. although even the worn S/H ones I have don't have any slack as such, just roll easier

To my mind there is no better way than precision ground just cheaper options. for such a small footprint precision rails don't work out to cheap as the carriages are the expensive bit.

bit more info needed as you might be able to get away with using only one carriage per axis if the design and use allow.........