Looking for some creative advice

My boy is chonky. So my problem is he is too big for his top perch area on his cat tree. He uses it, but poor buddy’s big back hangs off a little. If it were to be widened a bit then he would fit just fine. Anyone with a chonk of their own have a creative way to add some width to these things!? (They’re pricy so not looking for a new one, just a way to make it bigger for the baby!)