A little trick for barrel tenons is to cut your slot shallower than the base of your tenon. Then use a cold chisel to "raise" up each end of the slot. The chisel will also form the angle and need just a bit of cleanup. Someone used to make a special tool just for this but you can easily modify a hardware store cold chisel for this job. My tenon bases are usually .032" thick and I only make my slots about half the depth of my base thickness. Hope that all made sense.
