hand planing such thin pieces is not very easy.

If they tend more toward 3/8 you could get a set from each section.

If it were me, I would cut an oversize approximate scale, or maybe the smallest rectangle the design required. scribe a center line with a marking gauge and hand saw.

This way you are not ripping or planning the full 3x9 inch, going from a very tricky test of skill to an easy 2 minute delight in frugality.

A scale is thin and small, true, but now it is so small that you can mount the plane as a stationary tool, sole up and pull the material across the fine set blade.