@Huibertus, of course you can try to buy a BarButtons at https://jaxeadv.com/barbuttons/ but from what I hear he is not very responsive anymore. I have been in contact in the past and have build multiple BBs myself. I would be happy to sell you one (send me a DM if you want to). Yes it is a bit clunky, but it works fine and the buttons are a great fit to gloved fingers.
On the other hand, it seems counterintuitive to have your phone open, running the MRA app on your handlebar just to be able to operate the carpuride. I think you are better of with a AA/CP device that has a controller, or a phone to replace your carpuride, and a BT controller along with it.