Yeah, I thought about ways to make glue removal easier. Wax or tape before cutting the slot would make removal from the box easy. Not quite sure about what to do for the actual spline.

When cutting spline slots for miters, (usually on frames), I'll put tape over the corner so the cut will make for a perfect mask job. The glue on the spline doesn't matter since it is removed and the tape keeps the glue out of the box wood.

I'll just trim off close the excess spline on the band saw or with a small hand saw, then touch on the belt sander to get real close. Pull the tape and final sand.