Rats really like hammocks. We have tonnes of hammocks, our girls can go through them in minutes if they're feeling up to it
We usually have 3-6 in a cage at once, apart from Ashi's cage, he has one as he's not a fan of them, he prefers the bottom of his cage.
You don't have to buy specific "rat hammocks", we use old t-shirts, trouser legs, pieces of fabric, fleeces, placemats, etc. If they're chewers, they'll chew them no matter what.
Our big boys aren't chewers, they tend to get the same hammocks on rotation every week, the girls are though and we give them the same ones on rotation until they are utterly destroyed
Our girls currently have a cube that used to have one opening, it now has three and the bottom is half gone too, they have a plammock which they've opened to access the inside of and a corner hammock that is missing its corner. Ashi doesn't chew fabric but shreds his boxes!
Baby rings are good for hanging hammocks up, these are the best we've found, nice and thin. If you buy hammocks buy ones with eyelets in the main part as opposed to the cords, or just cut holes in fabric. This is what we buy in bulk, it gets used on the bottom of our two big cages and also for hammocks. Any cheap fleece is great though. If you're feeling crafty you can even sew it into a two-sided hammock or cubes and all sorts!
& thanks, my blue dumbo is my special but sweet Ashitaka :)