On my first one, I just used a razor knife & straight edge, worked great... (Second one I made from 1 solid piece of metal & punched holes (sharpened 1" square tubing) drove into a 'receiver'... or one could drill holes, waiting to see someone try that...)As far as length, make it extra long, you can also shorten... When I get home I can measure or I may have said how long in my video, don't remember....
Everyone's seat is different. From reading of people feeling they were sliding forward using the stock dimensions, I added a couple of inches (or more)to each end. That is working well, with none of that feeling. That said, from the top of my longeron to the bottom of the seat unloaded is 3 7/8 to 4 inches. There is plenty of clearance if the stick assembly is made per drawing..