One option is to buy a 2nd one, remove the strap clips from the one going on the front, and glue them together. That'll provide some reinforcement. Just make sure your glue is plastic-friendly!!
Another option is to mount your own brackets to the back. I attached Valors templates from his original batch of brackets for the Flexible Flyer sled-shield.
ABS plastic works pretty well for the cause. Sheets can be bought relatively cheaply (I got mine for under $10) and after you've cut out your bracket, it does heatform into shape pretty easily. I make my own functional longbows/recurves, so I already built a simple heatbox out of spare lumber, some aluminum foil (shiny side facing inward) and some small high wattage lightbulbs. If you know a DIY bowyer, this makes your job really easy.. if you don't, then you get to find your own solution to heatform the plastic brackets so that they'll keep a curve... Other options for bracketing would probably include 14 ply Posterboard.
My aluminum shield firmed up quite nicely once the metal brackets went on.
