Neil, if it were me, instead of starting with the Boxfish shape, I think I would use something like the "4-seat Cabin Arrangement" shown in Figure 15 on page 10 (hardcopy page 9) in the following pdf. This design uses boundary layer control and aft-body thrust and has been shown to have 1/2 the drag of conventional fuselages. Since you are planning on entering from the back, you could just make the door opening at the boundary layer slot. Leave the fan off around town, then turn it on when you reach 30-40 mph where the aerodynamic drag starts to become a factor. You could use a 3-wheel configuration or perhaps lower the nose area and/or flatten the bottom to have 4 wheels enclosed without too much drag increase, I think. This design has a lot of interior volume while miniminizing aero drag.
http://cafefoundation.org/v2/pdf_tec...4-2163-960.pdf