Hector my robo pees in his wheel. As he's a robo it's a obviously a lot less, er, liquid to contend with than a Syrian
but like Cypher, I just wipe it out & give it a proper weekly clean (If he was bigger & it was more I'd probably do every few days). Sometimes it needs cleaning more frequently because it sort of crusts up when it dries out so is harder to wipe away. But if you keep cleaning it & taking away his scent he'll be more detemind to pee there. But definitely try litter training him as suggested above. Cleo my Syrian used to pee in her nest & I've managed to get her to mostly pee in her sand jar now using the same method.
Just one thing though, you say it smells when it seeps into the cracks. Does that mean you have a wooden wheel? A plastic wheel would be easier to wipe down & wouldn't smell the same. And I believe that other members have plastikoted their wooden wheels & shelves to create a barrier as pee'd on wood will stink over time.