Thingyverse user Undeviljur created a 3D printable controller phone clip for using with the SteelSeries MFi Controllers.
I think it might work with other controller models as well, since clips that are sold for the Xbox 360 gamepads also work on the Nimbus.
So if you're a maker or just very cheap*, get your 3D printer running or head to your local makerspace.
iPhone Xs Max version & photo by reddit user coldblooded79 (thanks!)
*while the official Steelseries Clip costs around $15, compatible third party clips are around $3 on alibaba