DOn't bother trying to bodge it, it's not worth the effort and could bite you in the a**e if someone who knows what they're doing wants to buy the car. Have a look on Paddocks web site 'cos they have pictures of the repair panels to give you some idea of what they look like.
The inner sill is a flat reinforced plate complete with the chassis mounts, you cut out theold one and weld the new one in to line up these mountings.
The over plates are probably the outer sills which wrap around the inner, I'm thinking of using these because the rot is not near the A or B posts so I don't want to disturb them by fitting a full sill, but I will be cutting all the rot out first.