Buybits.com is a UK site with waterproof mounts for different Lumia devices. On amazon I found one for the Lumia 900. Supposedly the case dimensions can hold a phone the size of the 1020 from what they are stating.
There is a regular mount and a pro mount made for motorcycles. I figure if the pro mount would stay on a motorcycle, why not a mountain bike.
Here is a partial description from the page on Amazon...
Nokia has the 1020's dimensions listed as 130.4 x 71.4 x 10.4mm
The PRO mount is strong yet light-weight and fits handlebars with a diameter between 19mm - 35mm.
You will require a minimum clear space of 42mm on your handlebars to secure.
Twin anodized screws fix the mount to your handlebars, providing a stable base for the case.
Includes two liners to cushion against vibration and protect your handlebars.
Fits phones no larger than 138mm x 75mm x 15mm
Its called waterproof but the description says protects from splashes, so I wouldn't take it on a boat or go swimming with it.