Just make the shield's size its disadvantage.
Here are some pics from the LZ's public days.
The nice thing with that shield was its size and shape. Your limited your advantage of protection by having to protect your feet. Being small and wide limited its mobility and forces the users to be in uncomfortable shooting positions. You had some protection, but one slip-up and your exposed and dead, making it more fair then a 7 foot wall...
the cardboard idea is nice, but most real riot shields will take a shitload of punishment. And who wants to replace the sides over and over.