Retic trying to escape enclosure

[Squirtle]

How do I get him to stop? He has a screen top on top of his cage, and on top of them, I have weights. I'm not afraid of him escaping at all, but I want to make him stop so that he doesn't hurt his nose/mouth area from constantly pushing. I saw a picture of an albino retic before who tried to escape its enclosure, and the thing looked like it had big lips, wouldn't want that to happen to my retic.