I tack welded the template in place. Then hammered up the excess metal from the trunk to cover any gaps as my template was not perfect.
When I hammered up the excess metal it then fit perfectly. I then welded and applied a sealer ( its like automotive caulking) over the welds. This way I KNOW it's sealed and waterproof. I did the same on the inside of the trunk.
Tomorrow I will spray some rubberized undercoating just to make SURE it's all sealed and it will match the OEM undercoating. I could not spray it today as the sealer was not cured.