Ortlieb A5 sized document wallet.
Am on my second now, the first one having lasted me over 10 years through lots of running, riding, commuting, outdoorsy and military stuff. Used almost every day.
Costs about £10
Touch screens work through it, although bought the first one before this was needed.
‘Grippy’ rubber plastic means that it will sit comfortably between the waistband of shorts and hip without slipping when sweaty and running. This means no need to faff with bum bags and phone specific cases.
Texture also means less likely to fall out of a jersey pocket or slip out of the bag like it could with a sandwich bag.
Slightly bigger than most phones so no compatability issues.
Waterproof enough to swim with phones and valuables.
Also useful to keep passport, docs, keys etc in when traveling.
Texture also makes it easy to find when fishing around in a bag in the dark.
Can not recommend enough.