I customized the Huawei P20 Pro GoPro mount (https://www.thingiverse.com/thing:2853308 by Itiger013) to attach the Smartphone to DJI Mavic Pro Steadicam Handle. (https://www.thingiverse.com/thing:2491943 by kiwithreed)
I just put the mount on the long side, so you can use the phone in horizontal format. I think this is the best way, if you dont have the phone tripod mount.
It fits really well after printing, but just with the Silicone Case that comes with the P20 Pro!