I may try that myself, thanks for the tip. I found a link for a waterproof phone holder with bicycle mount, which will fit my Samsung Galaxy S5:
https://www.casebuddy.com.au/products/galaxy-s3-s4-s5-waterproof-bike-holder?variant=1074686216
One thing I don't like about my Garmin Edge 800 is that its odometer is rubbish. The odometer on my previous Garmin, an Edge 705, went up to at least 100,000km, but the Edge 800 odometer is limited to 42,949.7km, because it appears to store the distance in a 32-bit number and with too fine a precision. If it was stored with a precision of 0.1km, then it could go up to any distance I could possibly ride on a single bike, but some idiot at Garmin obviously decided cyclists would never ride more than about 43,000km on a bike!