Is whatever Winnebago used to attach the flap, still in place? If it is, this is easy. What I did, was go to Truckstops America and I bought an 18-Wheeler/Trailer Mud Flap. I chose T/A's because their flaps had all square corners, nothing rounded off. 36 high by 24 wide. Then I cut it in exactly half, giving me two new 18x24 flaps. A utility knife, against a straight edge, will cut this material. I was lucky to have a 1/4" leather/gasket punch and made the holes I needed with it.
On ours, the flap AND bracket were missing on one side. I made a new bracket out of punched angle steel available at Home Depot. This steel is probably out 18-gauge. You don't need the heavy punched angle used for shelving, hanging A/C units, etc. The stuff I bought "can" be cut with a good tin snips.
If you need a pic, I'll snap one and get it posted.
If God's Your Co-Pilot Move Over, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100 218" WB