I learned to cut lip slots with a bandsaw after I'd already shaped my lure when I forgot to do it while the blank was still rectangular. Doh!!
Take your time, trust your eye, cut from both sides, and know that you can adjust the lip once it's in the slot, but the epoxy hasn't yet set. I used tooth picks to shim and adjust the lip until it looked right to me. A centerline on the lip on blue painter's tape, up past the part that fits into the slot, helped a lot.
Also knowing that, worse case scenario, I could always use the bandsaw to cut the lip out again if it was wrong gave me the confidence to take my time and not worry about it being not being reversible.
I actually dry fit it first, and got my toothpick adjustments where I wanted them, so setting the lip into the epoxy went much easier.
I used wooden toothpicks, but plastic would probably have worked, too.