I constructed a table with the same dimensions as the observatory section of the trailer.  I constructed the base ring and dome on this table.  I drew a circle with the radius of the base ring on the table and laid up the sections of the base ring using this circle for alignment.  I used two layers of ring segments for the base ring and overlapped the joints.  After the ring was constructed, I bent 1/4-inch plywood around the outside for a splash guard/centering ring.  All these were glued and screwed in place.  I mounted 16 fixed casters around the base ring.  The dome will rotate on these casters.  I waterproofed the entire ring with fiberglass resin. After the base ring was completed, I cut out the center of the table.  This will allow me access to the inside of the dome as it is constructed.