Hey Simon. It’s ingenious but no, sorry, I wouldn’t buy it.
Tacx/Garmin already makes a perfectly serviceable tablet mount which I imagine costs far less than you could get the cost of a low production run unit down to. There’s even a way to adapt a phone tray to it, which I document here: Phone tray add-on for Tacx Tablet Bracket
If you want to release your design to the community via Thingiverse or Yeggi to let people tinker, I think that’s cool but this isn’t likely marketable.