I had one of my raspis in a shallow "tray" made of corrugated cardboard, folded and held together with ordinary white glue. (Though glue doesn't seem as cheap as it used to be!)
Some long bolts and nuts in the base in the right place, so the raspi can be put on the top, then bolted in with nylon washers to avoid any embarrassing short circuits.
(I have only Model As - these have extra holes where the network connector would be, so three bolts hold the raspberries down quite securely.)
Make it big & wide enough to hold a breadboard or backup battery pack or whatever you're tinkering with.
Also - at this time of year there seem to be lots of novelty biscuit tins & boxes in the local supermarket. All in the name of technology...