In my German book on building acoustic steelstring guitars I
also describe the building of a Dobro-style guitar. I didn't
use a soundwell but some posts placed between top and back which
significantly increased the volume of the instrument.
The usual way is to use a soundwell which can be glued up from several layers of plywood. The holes in the soundwell decrease the overal weight and also have influence to the sound. I wondered how this soundwell is installed and how there still remains access to a bolted-on neck. I called Fritz Harich a professional guitarbuilder in Austria (see links page) and here's what I learned:
The soundwell of his metal-body resonator guitars is glued to
the top and sits on four dowels which are glued between soundwell
and back. You can also make the soundwell as high as the inside of
the body and glue it to top and bottom. Metal-body resonator
guitars have the neck extended along the whole body running under
the soundwell. Therefore the need for the dowels.
Access to the bolts of a bolted on neck is easy through one of the soundwell holes (green arrow). The resonator cone sits on the soundwell and the center of the soundwell is scalelength plus x (see placing a bridge) from the nut. By turning the spider bridge a little bit you can achieve a slanted compensated saddle.