Cannot get him to stop digging in his litterbox

So I recently got an xpen for Briar so that he would have some room to hop around and stretch out when I can't be home to let him out, but for some reason even though he has room and toys he will not stop digging in his litter box. He even still uses it fine and hasn't had a single accident, but I clean his box turn around when when I turn back hes dug half of it out already.

I use those cylindrical paper pellets for his litter box. For toys and such he has a digbox, a towel, a plain brown paper bag, and a soft towel to play with but apparently his little box is just so much more interesting than anything else x.x

Do you put hay on top of the pellets in the litter box? That could minimize digging because he'll be more interested in munching on the hay. (Don't worry. Rabbits won't eat soiled hay.)

I can give that a try, thank you <3