Ima Bass Ninja

I've made quite a few Pretty easy to do. I used a lightweight nylon type material. Basically you want to measure the distance at the bottom for you rod ( casting and spinning will be different). I made mine with a lightweight nylon material. Take whatever material you want to use and fold it In half. Measure the size base you want ( mine was 2" ) and then to a point with the length you want ( mine were 5'-6'ft). Cut a straight line from you base point to the point. Sew the loose edges together and flip inside out. Sew a piece of elastic on the bottom opening to attach to the reel (this will help it stay down) and you are done.
Having said all that I pretty much just buy the rod socks now because by the time you buy the material and put the time into making them (about 20-30 min when you get good at it ) it's just easier to buy them.