If you've ever seen or picked up Koltin's shield, hopefully you liked it even if the handle is uncomfortable, you may have noticed that the core is plastic. High Density Polyethelene as a matter of fact, and its very nice stuff.
If you have the money to do it, I highly recommend purchasing or otherwise getting a hold of said plastic core. You could find a curved shape out of a pickle barrel, but if you wanted something flatter, I'd go with a smooth sheet. You can find them at mcmaster.com, part number 8619K435.
Enjoy! ^_^