I guess you could use a flat sheet but there needs to be an access hole cut for the fuel sender unit, and make holes for the rear seat belts
 
Some use check plate but boot floors aren't expensive so just buy a new one for £45 or whatever. If your side panels are ok you can bond them back in with butyl sealant or you can use bolts. BUT check with your MOT bod first as some question it (even tho its the side panels that give the seatbelt its rigidity not the floor).
 

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