Supports for egg tempera come in a few flavors--hardboard, wood panel, and plywood. Hardboard is tricky as only the untempered is considered archival and it seems that most these days have some kind of treatment. Wood panel is cool as long as there's no splitting. Good quality plywood, and probably with some backing support strips, sounds to be ideal.
I have two supports coming in from Natural Pigments. The birch panel is actually a sheet of birch plywood with backing support strips. Here's a picture that links back to the catalog. It's a real beauty, isn't it?
Along with this panel is coming a half dozen gesso boards. These are untempered hardboard fully coated and ready to go. Well, almost ready...
Koo Schandler's post on the EggTempera site explains that gesso boards need to be carefully examined for scratches and pin holes. Light sanding and wetting to smooth the pin holes should take care of that. Next the edges should be chamfered to prevent chipping--NP's board come chamfered so that's done already! Lastly, the board needs degreasing with a wipe down with denatured alcohol.