You'll need two more bolts and washers per side, just like the ones that hold the front flaps to the bracket.
At the hardware store, they're called lag bolts(very coarse thread). They usually have a pointy tip, though.
Use your rear mudflap to locate the pre-drilled holes under the undercoating.
If the lag bolts strip out the holes, you can just go a size up with nuts and bolts.
Best,
Shayne.