First off, you're already breaking one big law and that is on license plates -- I can't see it from either front OR back. This is bad and easy pickings for a ticketer. Next is looking or the appearance of security/stability. If it looks flimsy and could come off at any moment, this could provoke a pull-over and some explaining to THE MAN. Plus with so much cardboard, you'll get attention like basjoo does.
For testing this is fine but make sure to do this on some light/no traffic area that gets little attention from the police and make sure you either cut out some spots for the license plates to be seen at distance or move them into the windows (check your local vehicle codes for allowability here). I would also slap on MORE MORE MORE TAPE. I'm a big fan of blue tape (
) but under stress at speed, they suck at holding onto anything... at all. Plus over time, they naturally lose their adhesive qualities and forcing it to hold on to cardboard just means it will come off sooner than later.
(this is why I made my rear skirts and front grill blocks literally out of just tape -- I figured they were small and light enough that if it does come off, it wouldn't pose as much of a threat as if a piece of cardboard flying off at 55mph. Plus tape on tape isn't exactly forcing the tape to work hard holding on to the car and itself)