If you're set on using rubber, you can go here:
Racing Rubber Lip and try using the lip for a 88-91 Civic, it's about the closest shape to your Geo.
EDIT: you can also look here:
Sheet Rubber for ideas.
I have emailed the owner of RubberRacingLip.com and invited him over to EM to offer his product to us as well as planted the idea of making rubber side skirts. Let's hope he makes it here.