Masonite (sometimes called hardboard
) and pre-primed masonite can make good supports. There are some wetcanvas.com threads that describe the process.
This video shows a very simple approach, though it is probably better in the long run to seal the panels completely (front, back, and sides) before painting. After the painting is finished, it is best to apply an isolation coat and final varnish.
But if the work doesn't have to last forever, or for 'fun' projects, this approach is probably fine, at least in the smaller sizes: