Make a sleeve to hold it.
What you need:
– 1 litre squash bottle, high juice or similar
– Sharp knife (and protective gear if you aren’t handy)
– Lighter / olympic flame
What you do:
– Cut the top off the bottle with the knife (wearing protective gear if you need to)
– Melt around the top / cut with the flame
In summer I often use high juice bottles instead.
In heavy rain your sleeve will collect water, so you might want to also pierce a few holes in the bottom.