Here's a good idea for DIY speedometer and tachometer cushions:
You can use a large O-ring like the one off of your old oil filter. I used the O-rings from a couple old 500/550 oil filters for my 350K4. They're perfect for the early style gauges. You can also try vacuum cleaner belts. I used a few of these on my 500K1. Both keep the gauges secure and look pretty good as well.
Later style gauges are a little larger, but I'm sure you could still make it work. Here are some pics (the 1st is the 350 with oil filter o-rings, then the 500 with vacuum belts, and last is stock 550 just to show the difference in the gauges:
