When I was first getting into downrigging I looked at making my own. Once I priced out the materials, my time and the broken knuckles like Set_The_Hook, I determined it was a wash and purchsing on that has had the bugs worked out of it was a better option. Now if I had just headed the advice of purchasing an electric right of the bat, I wouldn't have had to upgrade several times.
A few years ago my son-in-law built one for an old boat he had at the time. It worked for his purposes and most of it was made from scrap parts. It can be done. The original downriggers were homemade so there is no reason to think things are any different now. As for line, spools of downrigger line are available and now many are using heavy duty braids for the line. One problem with homemade is there is no slip clutch in case you get hung up on the bottom. Figuring depth can be done in a number of ways. I believe my s-i-l used a simple linecounter you would use with any fishing rod.
Years, (and years) ago I made one out of a piece of angle iron that I clamped to the side of a little 12' aluminum boat, to which I attached a 50' length of 1/4 rope, marked off in 10' intervals so I would know about how deep I was. Hung a 10lb weight on it, and hooked up a release. It worked, but not very well. And pulling it up by hand, with no reel of any kind was a chore. But, I was young, strong, stupid, and broke, and it was better than nothing.
I made one about 15 years ago. After the handle got away from me, I almost needed stitches. My design was Terrible.
That same season, I saw an all aluminum homemade one. It was beautiful, well designed and safe.
I would recommend dropping the $50 or so for a used cannon clamp on.