+1 for the milk tub diffuser....don't buy one of those, buy 2 litres of milk, when you have eaten your corn flakes (am I allowed to say that here? Advertising? Other breakfast cereals are available.....!) wash the tub out - use cold water so you don't seal the protein from the milk to the material.
Cut out an area of one of the sides suitable to cover your pop up flash. Make it a bit bigger than you need. Put a couple of holes in it with a hot skewer or large needle, or a drill I suppose. You now have the same as you were about to buy. A coupe of cable ties should give you a strap at theback to hold it in place, blu tac might work too, but the waxy surface of the plastic might not like it and refuse to stick.
Go and shoot pictures with it to keep you satisfied while you save for another flash and a Lumiquest 80-20 kit. Now that IS worth the £40 or so...used one for some time on my flashes. Before that a normal white handkerchief put on with a rubber band softened the Metz on the medium format camera!