Good find on the 12x20. That specific dimension is what GMAC allows and it's genuinely hard to source.
On test conditions: the whiteboard thing is only one piece. A few others that made a real difference for me:
Do all your practice in the same physical setup. Same desk, same lighting, same time of day if possible. The more familiar your environment feels, the less cognitive overhead you're spending on comfort and the more you have for the actual questions.
Use your whiteboard from day one of timed practice, not just for mock exams. Sounds obvious but a lot of people practice on scratch paper and then switch to the board for their test. They're different to write on and it can throw your layout habits off.
Practice DI questions with the calculator on screen, not a physical one. The GMAT Focus gives you an on-screen calculator for Data Insights and you want to be fast with it. The interface has a slight delay from keyboard and mouse input that trips people up when they're used to a handheld.
No notifications during practice blocks. Full airplane mode or a dedicated device. Even seeing a notification banner for half a second costs you attention. I know this sounds obvious but I had a 20-point mock score swing when I finally took it seriously.
Good luck with the prep.