Hi. I bought the 335 with the tray and strap from the Club shop:
https://vwcaliforniaclub.com/shop/product/thetford-qube-335-porta-potti-package. It fits in the right hand part of the cupboard and fits well on the tray. The one thing that is a bit of a fiddle is moving the shelf to make the space for it. The guide that comes with it talks about drilling new holes and using T5 type shelf supports supplied (there were none, though). Fortunately, there were already pre-drilled holes that were high enough. After taking the four shelf supports off (avoiding levering them at the top to avoid breaking them), I fitted the two on the left. Then I placed the left of the shelf on these, working from below. Once seated there, I pushed up the shelf on the right. Once I was past the right hand side holes and while holding the shelf with one hand, I put each of the supports on that side into their holes. There's space than before on the shelf, but still some available.