If it's a non-structural panel, (and I hope it isn't if the jack is all it takes to punch through!) here's what I'd do, given that the car probably isn't worth a big dollar welding repair (most of which will be removing carpeting and interior around the weld area so you don't burn the car to the ground)(and don't forget how vicious flammible some undercoat is, like creosote). First, you should hammer the panel flat or back to it's original shape, leaving a seam or tear to fix. Then you need to mechanically re-join the metal along the tear line(s). This can be done by welding, or use patch pieces of sheet metal and pop rivets. I cut my patch pieces to size, then butter the area where it will lay over the floor metal with flexible seam sealer. This stuff is like glue, stays flexible, and does the job of waterproofing the repaired seam. Fibreglass will crack after time and leave you open to water penetration, but it's an option I guess if you are careful to clean the metal thoroughly. After many years of fixing rust holes in old floors, I've gone over to rivets and seam sealer, since it's easier, faster, and the repair usually outlasts the surrounding metal. You don't have a rust issue here though, but flexing of floor metal is always a problem, so take this for what its worth as an option.

__________________
1986 560SL
2002 Toyota Camry
1993 Lexus