Just for informational purposes only, the attachment I have looks like this
There are two blades set at a fixed angle to each other. Adjusting the depth of each one is a pain.
To keep it from scuffing, I tried padding the narrow, flat surfaces outside the two blades with release paper, but they kept slipping off.
If you could modify fletcher blades to fit into the OvalMaster holders as Pat suggests, that might be the way to go.
With the OvalMaster you might try creating a V-grove by scoring the back of the mat, flipping it over and scoring the front of the mat.
For example, if you wanted to cut a V-groove 1” around a 5 x 7 oval, you would set the lower “C” scale at 5” and the upper “D” scale at 2”. That will give you your window opening.
For the V-groove, leave the upper scale at 2” and set the “C” scale at 6”. Make the cut, then tape the fallout back into the mat. Flip the mat over and cut the back of the mat. Then carefully tape the fallout back into the mat.
You will have to be very precise in your measurments, and you will probably have to fiddle with the dimensions a little bit, though, by reducing the “C” scale to 5-7/8” or so for the back cut. I’ve never done it, but in my head it seems that it should work.