Apple watch bands are also made out of silicone and were super popular engraving items, so searching for info about them might help. This will get you started. 
But it’ll still take a little testing to get it just right because your bands sound much thinner than a watch band, so if it were me, I’d get a few extra to play with. Maybe start a little test engrave at one of the recommended watch settings, and if it’s too deep, back the power down a bit or up the speed and try again. If you just do a little test square, you can probably fit several tests on one band until you get it right.
https://community.glowforge.com/search?q=apple%20watch%20bands%20settings
And using a jig to hold them flat helps, and I suggest using set focus and confirming where set focus hits on the bands, because if your material is narrow, set focus may miss the band and measure in the wrong spot. (Watch to make sure the little red focus dot hits on the band, and not on the tray.)
Or instead of a jig, you could use a holdown pin. (If you don’t have these yet, they’re a game changer and must-have!) This is my personal favorite version, but there are several good iterations.
Good luck and let us know how it turns out!