1Laser Cut piece and 3D Printed Pieces
I've included the .DXF file for laser cutting the main body plate on my PinShape page: https://pinshape.com/items/30015-3d-printed-r2pv1-omnibot1-kit
The idea is to 3D print a couple of parts for mounting the wheels and sensors ... but I have designed many other robot parts that can be used on this kit: https://pinshape.com/items/33930-3d-printed-standard-servos-pan-tilt-brackets-kit
or change the wheel size: https://pinshape.com/items/34077-3d-printed-60mm-70mm-80mm-85mm-wheels
Build the base plate up with the servo motors and 3D printed parts ... add your RoboGuts™ circuit board and speaker to begin experimenting with ways to have the robot travel around in specific directions performing simple tasks.
I'm hoping people ask for other 3D printable components so that this robot platform can perform more complex tasks ... http://www.r2pv1.com/
2Making a base plate from cardboard ...
To make a cardboard base plate it is either finding good strong thick "TriWall or QuadWall" cardboard or you can laser cut a few layers of standard cardboard box sheets crossing the grain so that when you glue them together (laminate the plates) they will become much stronger. The base plate thickness isn't too important so two or three cardboard layers should work fine.