Thank you all for the compliments. I like making things, and I've had lots of time to prepare for my guineas, so lots of my creative energy has been going in that direction!
Salt n Peppers mum - I took two fleece blankets, cut the spotty one to the size of the cage base and the pink one into strips twice as wide as the correx is high ie. 30cm wide for 15cm correx walls. Then I sewed the long pink strip to the edge of the spotty rectangle so the pink goes up the sides and over the top of the correx. I put two massive towels in the bottom of the correx tray then put the fleece on top and tucked the edges in between the correx and the grids. I'm not sure yet if they'd be burrowers, but this way they don't get the chance!