re: Spline material

I agree with Chuck's assessments, watch out for "chippy" woods (oak is notorious), but any thing will work. End grain showing so no good way to blend in, but the contrast will make them a feature so careful layout is needed.
I always use tape over the corners before cutting, makes for a perfect masking to protect from squeezeout. Excess is trimmed at the bandsaw (as close as I dare), then either the belt sander or sanding block to level up. Corner to center only to avoid flaking off the "peak".