The production version of the Warbl (invented and sold by Andrew Mowry of Bend, Oregon) has its own long slim PCB, but I’ll be using the ItsyBitsy 8MHz 3.3V prototyping board so my version won’t fit into a slim whistle-like housing like the production Warbl. The ItsyBitsy is 0.7 inch wide (it’s American, so inches.) This is 17.78mm so I’m going for a tube with 20mm internal diameter and hoping it will fit. I’ve ordered two tubes: one is aluminium for £9.99 and the other is transparent PVC for £6.99, both prices including tax. I’ll see which is easiest to work and which is nicer to play. The length of the PVC tube is 50cm and the length of the aluminium one is 30cm. 30cm is a good length for a whistle.