I am not sure, how good that works, but maybe it could help as well, to place a copper-plate behind the phone, as copper is a very good heat conductor; plastics is not.
In addition with some ventilation, this might help dissipating the heat.
Especially bad might be a holder with a plastic cradle, that is covering the phone, thus insulating it and accumulating heat.
Please keep in mind, I am no physicist and it’s just kind of guessing. (Though coolers in computers are made of metal and cooling fins are used to enlarge the surface if I got it right.)
A copper plate of 2 mm sized 70x135 mm e.g. is 8.18 € from this German shop:
And here’s an offer from “banggood com (China)” 100mm * 150mm * 0.8/1.0/1.5/2/3/4/5mm Pure Copper Sheet Cu Metal Safe Using Guillotine Cut Sheet Plate - 5mm