That was design to specific racing class rules. That's why it has four wheels.
One Ecomodder member did a CFD analysis (FWIW) of a feature that would fit it. A bellmouth diffuser.
I haven't found the thread so I can't pass along his results, but it was similar to this one: a gravity racer in Barcelona.
ecomodder.com/forum/showthread.php/making-more-efficient-aerodynamic-soapbox-gravity-race-car-32441.html
or this one:
ecomodder.com/forum/showthread.php/aero-teardrop-its-side-28652.html
This one is notable for Permalink #31
https://ecomodder.com/forum/showthread.php/gravity-racer-gallery-extreme-aerodynamics-1208.html
I'me seriously going to find a thread to repost
the graphics from Permalink #31 (dates to 2008).
I think it's in the near future also. Mainly because CFD will be implemented with OpenVDB. It's already being rolled into Blender with its physics engine. Coding it is above my pay grade unfortunately. I used to work with programmers. I stand in awe.
Especially Ian Hubert and Dynamo Dream.