Below is what I did when reinstalling the belly pan of the http://www.airforums.com/forums/f219...ign-14737.html
1" X 1/8" aluminum strips are held in place with Stainless machine screws.
Below are the repairs and supports in the area of the step.
A material I used with great sucess for sealing the underbelly was found at NAPA (automotive parts). The product is listed in NAPA books as "evaporator insulation tape". It is a black mastic sold for sealing the area of the AC/Heater Plenum where the AC lines and the Heater lines enter and exit. It is also used by Home AC techs for sealing the same area and filling in larger gaps in the AC/Heater duct system. It comes in rolls about 3/16" thick and about 2" wide - not cheap, but it seals great and is extremely workable. There is no sign of it drying out after two years - still pliable. I worked it into any place I could find that might provide an entrance point for small uninvited guests.
The rivetting of the aluminum angle to vertical supports mentioned earlier in this thread is also a great hint. Shorter sticks of aluminum angle are available at most hardware stores and big box home centers. Not the cheapest way to buy it, but usually available when you need it.
I agree with craig's (davidz71) comment about (not) fully sealing the belly pan - I still have plenty of "breathing" gaps - definitely rust proof anything you can prior to replacing the belly pan.