Go to Lowes, Home Depot Et-Al and get a 6 or 7" square plastic fence post with caps. also a hinge and a bolt latch.
Now attach the post to the top of the bumper.. I won't go into how to do this but if you use bolts use carrage bolts (Smooth domed head with a square shoulder) from inside (no sharp surfaces inside post)
One end glue the cap on
other end you will need to slightly modify the cap and end so you have a "Tongue" that sticks through a Slot in the bottom of the cap. Hinges on the top.
Hose goes inside, ends fit too.
Close the "lid" and drop the latch bolt through a hole in the "Tongue" of the post gravity will hold the bolt in the hole and anyone who steels the hose.. Well... Would you steel THAT item?
Or you could padlock it but I think that's over kill
Amazon dot com example of the bolt latch you use only the big part. 
