I agree with the Poplar for painting. Great to work with. Few knots to
deal with. Very little movement.

If you like the MDF idea you can use Armorcore (sp?). It is MDF
sandwiched over veneers of ply. A lot lighter than MDF proper. Of
course, this is ply like but honestly I don't see the problem with
using ply for a painted piece.

BW