Poplar.

Second the poplar -- it's cheap, knot free (for the most part) and takes paint well.

Third for poplar. It's also available at many home centers in a variety of thicknesses.

poplars cool unless you want to stain it (poplar takes stain a little funny sometimes)

Poplar takes stains funny a lot of the time. Especially the stains you find in big boxes.

I used some leftover aeromatic cedar from a closet lining kit.

Aromatic red cedar contains oils that inhibit curing of many finishes. It may work, but I'd test first with your intended paint on scrap. No sense putting a lot of work into this and ending up with finish that is permanently soft.

Here is a link that might be useful: see adhesion problems