Eddie,
I don't think any provisions were ever designed in to fasten the lower panel down. The idea of the previous poster to increase the bend past 90 degrees will help a lot.
Also, notice that the two holes behind the lip in the center of the fuselage have another pair of holes just under the lip where a person could insert bolts with fender washers (or similar fabricated doubler) to clamp up the edge. These 4 holes - two behind and two under the edge are for trigear applications where a bearing block is bolted up on each side - with the trigear, the bearing blocks hold the bottom of the firewall lip tightly. If you have a tailwheel - those holes are not used for anything else and could certainly be used to resolve the clamp up issue of the firewall lip.