I’ve worked up the plan to my satisfaction and it currently being proven by someone who I know will upgun it to the point of needing to be modified [sigh] sailors.
Anyway It takes under ten metres of 4X2 to build one along with screws, coach screws, a spring shackle, a few other bits and pieces and most expensively some nylon line. You can use cheap plastic rope but its just not very powerful. I’ll be using some retired absiel rope if i ever get a chance to build one myself.
Just a reminder this is not a huge stone throwing weapon of destruction. It is an educational project with the ability to throw a tennis ball in a statisically useful way. The plans include a couple of mathmatical activities to aid in making it interesting and useful. All this can be yours for the bargin basement price of $10. If you want to be notified when the plans are ready for distribution just let me know in the comments or email me.