Great information about the GPS mount removal.
As far as adding a Phone mount - check out SP-Connect. They are a bicycle-based phone mount that also has motorcycle mounts. https://sp-connect.com
You basically buy either a handle-bar mount, or possibly a adjustable bar, ratcheting mount or whatever fits your specific circumstances - and then buy a phone case that fits your cell phone.
Their phone cases have a "rotating cog" on the back that allows the phone to either rest portrait (upright) or landscape (horizontal).
Search the site and find what works for you. They even have waterproof covers for the cases.