Ah, the big-bum-bunny problems. I feel you. I struggle with finding affordable/durable hideys for my 5.5kg bun too. He won't fit into places my 3.6kg girl can fit.
The stores seem to never carry stuff purposefully made for the big guys, so we often have to repurpose something else to fit those needs.
I found that most often, huge plastic bins with a side or two cut off work quite good as both hiding in and hopping on. I have a 60*40cm bin that stores some of my fabrics and my big boy loves to hop and scope on it. I secured a piece of cotton fabric with the lid so that it'd have better grip on it.
We also had an old laundry basket (roughly 50*50cm square of 80cm length) lying around-opened up the bottom and the sides, pushed the lid against the wall and done-its wooden frame is sturdy enough that i could attach a thin layer of plywood and a small ramp leading up to it. Thus, it's both a hidey and a lookey. One of the few things that still stands without being destroyed.
On a similar note to diy-ing, perhaps you can find some sheets of plywood, nails/hammer or screws/screwdriver and assemble your very own hidey rectangle to the size you need.
If your buns don't care much about eating cardboard, i've seen that huge boxes of those work excellent, are relatively cheap or even free, easy to work with and recycle if meddled with beyond repair.