Hello John.
Yes, making a fuselage plug the way you describe it is quite feasible.
I only made the front part of my plug with planked formers, the rear part was made with sections of hot wire cut foam blocks sheeted with balsa. (In principle the same method as you use to make foam wings. Most of my plug was made in two halves, i.e. left and right hand side, and glued together before glassing. The whole plug was glassed with one layer 6 oz cloth and one layer of 2 oz cloth, applied to one side at the time, overlapping at the top and bottom.
I have enclosed a few pictures of my process.

Regards,
Magne