Slightly more fiddly, but you could cut small bits of plastic into the shape of the button you want, then glue onto a slightly larger piece of plastic to create the tophat shape. Some hobby shops have a reasonable selection of plastic shapes, usually in white styrene, or you could do acrylic as well. A little solvent glue will hold the pieces together permanently. With care, and a sufficiently thin backing sheet, you could even have multiple buttons glued to one backing sheet (Ideally you could use the front panel as a jig to hold the button faces in position while gluing, just be careful not to glue the buttons to the panel!).
If you've got access to a laser cutter, or are willing to use one of the laser cutting services, you could make arbitrary button shapes, and even engrave them with legends without too much fuss.