I learned about
these cupholders on the Mercedes site I used to frequent...IIRC they were essentially the same as the factory design on the early 2000's Mercedes S-class cars. These can either be mounted on the floor/center console or on the door panel or other vertical surface, depending on what you want...I had two of these on the front door panels on my first M-B, and I was very happy with them! They'll hold a 1 litre water bottle without any problem but can also hold a small cup, and I never had any spills!
I would recommend using stouter hardware to mount them, though...they provide sheet metal screws that will do the job, but I instead used through-bolts with large diameter washers to spread out the load...never had a problem
Please post pics of whatever you end up doing...would love to see it. I haven't made a final decision on how I'm going to set up my cab when I'm done with everything, although SOMETHING certainly needs to be done...