The Samla tapers quite a bit and is actually smaller at the base than the box you linked (the length is about 67cm, not sure about the width). Of course you could add lots of bedding to maximise the floor space.
Asda do a box that's 84x54 at the top, and doesn't taper too much. It's out of stock at the moment.
https://direct.asda.com/george/home/...kUpv3gVm3kwLX0