As a step in the right direction, use an upcut to allow for good chip evacuation - you don’t want to smash them down into the material you are cutting in this scenario. You could even just use a 2-flute 1/4" endmill as a start. Single flutes are harder to find but 2 flutes are pretty common. Try drillman1 or Rogue Systems Inc. on eBay for some inexpensive selections.
These are two good options:
Edit: After searching a little harder, I’d highly recommend going with a single flute if you can.
These are really nice and I use nearly identical ones in polycarbonate frequently.