Best way I've found for slotting wood is with a table saw and a good miter gauge. Have to make a jig.
Need 2 peices of 2X4 or something that you know are good and square. Cut one 2 inches shorter than the other and screw them together so that they're flush on one end.
Using a large C clamp, clamp your peice you want to slot into the jig so that it butts up agains the end of the short peice the is back tight against the longer one.
If your blade is good and sharp, you have a sturdy miter gauge to clamp the jig to, and you take your time, you do a surprisingly good job ripping slots into thin peices.
For a friction folder I guess it would be a 2 part process. First slot the depth of the handle lengthwise to keep the solid back.
2nd turn the handle on end and raise/lower the sawblade to cut out the front of the handle where the blade sits.
Another option would be to start out with a wedged shaped peice and slot it lengthwise.