You have done a good thing.
Step 1: The Inspiration.
One day I found some fluorescent powdered chalk in a skip.
I decided to mark every poo I encountered on my jogging route, as I ran.
The next day, when I jogged, I easily avoided every one of them.
Also, I had the humourous vision of people looking at these turds and wondering who would have been sad enough to annoint them.
It was me.
Anyway, then my mind came up with a bike mounted spray system.
I didn't want to make it but my brain just wouldn't shut up until I did,(I'm sure you know the feeling.)
O.K., I didn't really have a plan except to make it a simple and quickly as possible.
Here are the bits that I started with...
A brake lever, Aluminum pole, a bit of wood, and some brake cable.
Also pictured is a stencil; the idea was to turn a thing of revulsion into a thing of beauty.
Unfortunately, tests showed that the stencil would have to actually be laying on the poo to be effective; I gave the stencil idea up.
Step 2: Step 1.
Wrap some inner tube around your spray-can.
Cable-tie it together.
Step 3: Step 2.
Drill a small hole in the pole, (also for the cable).
Affix on the pole, quite loosely.(I used a folded inner-tube as a washer, to give some flexibility.)
Step 4: Step 3.
I wrapped the pole in inner tube and used a cable-tie and a fork-boss.
Step 5: Step 4.
Attach brake cable to pole.
Stick on a bit of tape to keep everything on track.
Step 6: Step 5.
(I have used a block of wood to spare your delicate sensibilities).
This does work; the riding action mixes the paint.
If you are as sad as me , this adds a certain pleasure to a routine cycle ride. (Such as cycling to work.)
I'm sure that line-marker paint would be brighter but it's just too expensive over here.
Step 7: Epilogue.
It has potential to make huge works of graffitti art.
It has potential to surreptitiously spray cars that have been naughty to you on the road.
I would hate either of the above to happen but there you go, people are morons.
Anyhoo, thanks for reading my drivel and hopefully my brain will shut up now.