Joystick testers are certainly nothing new so based his design on an already existing one, specifically http://www.mks.purespace.de/technical/c64tester.htm. So, if  you feel that prototyping boards and the like are too unreliable and finicky to use in practice you can order a set of my PCB to make a more permanent version of it - if you like and prefer prototyping, then there's the page to start with.