Ultimately water proofing doesn't really work especially if it is a hard wood. You can use boiled linseed oil but you would ideally need to soak the piece in it and then final sand it once its dry (note plain linseed oil will stay tacky). Since you dont need these to be food safe you could use a staircase sealing oil which would probably need about 5 coats to be sure and since it is designed to be walked on it should be hard wearing. Finally you could use tung oil but like linseed oil it would need to penetrate the wood and you would have to apply several coats and ensure that each coat dried before sanding for the next one. Sand at about 300 grit between coats final finish at 600 grit. Are you going to fill them with solid soap?