After examining the stiffness in the floor I decided that a stiffening rib as the best way to stiffen it up. The other option I considered was adding a foam or honeycomb core. The core is more work and may not be lighter. The stiffening rib would go from the back edge of the wheel housing to where the rear suspension pivot is mounted. The rib along with the very stiff corner at the bottom edge of the shell should act as a frame on either side of the shell. If it works as well as I think it will the drivetrain should be very stiff. This should help with climbing as there will be no losses due to flexing. Here is a picture of the pattern I am developing for the stiffening rib.
Another one with the rear suspension taped about where it mounts.
Other velos seem to have an aluminium frame member running under the seat (Mango) or an aluminium extrusion running in the chain trough (Quest). Hopefully my solution works better than either of these methods. If the stiffening rib is strong enough to step on beside the seat you will have some options for getting in and out by not being limited with only putting weight on the the bridge between the wheel housings. There will be another rib bonded to the side of the shell from the bottom corner to where the crossmember from the fender ties in with the sides.