An M12 wingnut and bolt taken from the McMaster-Car catalogus. Printed with PLA and scaling the nut to 103% and the bolt to 97% gives a perfect fit when printed on the Ultimaker 2+. You need to use an allen wrench to drive the nut through the bolt the first time but after that the fit is perfect! What a way to print objects. Just download from McMaster-Car (https://www.mcmaster.com/) and print. Wow.