My rats have sharp claws, and a brick in front of their water bottle doesn't help!?

One of my rats is a double rex (hairless) so I new to be extra careful with what I use to file their nails so I don't cause abrasions on her skin. Would it be safe to get a pumice block to place in front of their water bottle instead? Can I maybe put a very fine grain sand paper on the ramps in their cage? Any suggestions will be greatly appreciated! And please don't say to clip their nails... I've tried and failed miserably and my girls held a grudge for a day "/