This is my on-bike setup. It show my GPS, mounted on cheap aero bars, and my clipboard, which is mounted on a cheap, modified, strap-on rear fender, which is strapped to my handlebars. Had to do a little cutting, and added some screws, but it holds my pencil and papers where I can see them while riding.
What it doesn't show, though, is my paint can in a water-bottle cage, mounted normally. I also carry a backpack, which has nails and washers, hammer, duct tape and sidewalk chalk, spare pencils and Jones counter, and surveyor whiskers (stake-chasers) and spikes, for when I want more visibility for my marks. I can put gloves, snacks, an extra can of paint, and anything else I want to take along in the backpack, also. I put my phone/camera/calculator/voice recorder in my cycling jersey pocket, so it is handy.
One final "tool" I use is airless tires. They cost the same as a normal tire and tube, and they don't go flat. Don't want to be measuring a marathon and have to start over, due to a flat.