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If you want something to still have tube attachments for playpen access, there is also this (it's only really some Ferplast models that have tube access). It's a bit on the narrow side but a good length and a good deep perspex base at the front. It's also a bit expensive, but that price does include the wheel. Robos need a while 8" diameter or larger. Shame the photo shows it with hardly any bedding in!
Ferplast Multipla