After experimenting with corn and potato starch, corn star is no good. But since I exhausted my supply or corn starch, I know a few tings, still workable as a fleshlight.
A fastfood plastic cup is about 500 ml, I add 8 to 10 scoops of corn starch, 2 tsp each scoop. I think it's 15 ml per tsp, so theres 240 to 300 ml of starch. The easy way is add cold water to starch, stir thoroughly to dissolve completely, and then heat it while stirring. At the end it will be a very very thick cream. I think there's a saturation point where the cream can't get any thicker no matter how many starch or how long you heat.
Pour the stuff into a cup and put a PVC pipe your size in the middle of the cup, all the way to the bottom. Wrap the pipe first with cling film to cover the pipe opening, and make it easier to take out the pipe.
Cool for 30 min or much longer. You can reheat using the microwave when use.
I think the bonding of corn starch is not as strong as potato starch. So for corn starch you get a paste or solid. The solid breaks into pieces when you squeeze it.
I think potato starch is better - it remains flexible, and do not break into pieces when you squeeze. But I didn't do as much experiment as corn. That's pretty amazing as the fleshlight is a pipe like structure (although supported at the outside by a plastic cup), it has to stay in one piece when you abuse it, and have to be flexible and soft!