I think there are a couple ways to accomplish your goal. Please understand these suggestions are VERY general, but should get you started:
1. Using the BACK of a #11 blade, carefully scribe the area you want to open. When deep enough, carefully use the blade portion to cut through the scribed area. Clean up with some sandpaper.
2. Using a small drill bit, drill a pilot hole in the center of each eye. Carefully enlarge the hole with your X-acto (or some needle files/rifflers) until you achieve the dimensions you desire. Use some sanpaper glued around a stick (or a needle file) to achieve the final shape and remove any rough edges.
3. Using a small drill bit, drill several holes around the periphery of the area you want to open. Then use a small saw blade (razor saw) to "connect the dots". Follow up with a small stick with sandpaper glued around it to achieve the final shape. (Again, you can use needle files or rifflers for this.)
For saw blades, I believe Micromark makes a set of blades that will fit your X-acto handle. They look like #11 blades except they have varying types of teeth--coarse to fine. I have a set, and wouldn't be without them. Micromark also sells needle files and rifflers; they are quite handy to have in your toolbox.
Hope this helps a little.
Gip Winecoff