Thanks for the feedback, and great photos. I used the rubber mat in the bottom for sizing it, thought, but didn't realise how big a gap either side that would create.
I can make changes in Fusion 360 easily if you can give me some measurements. Looks like you can get a straw either side, so I guess it's what the diameter of those are (unless they're a bit squished, plus how much I need to move the step backwards. I could also make a longer version with larger cup holes, I based the holes on the maximum of all the coffee cups/water bottles we had in the house.