the king of stockley

Has anyone any knowledge of where to get a tonneau cover for a 110 pick up,I have found a genuine product (£250) but hoped for something cheaper?

I have a 1984 110 with a perkins conversion soon to be 200tdi .It has the original gearbox LT77 and has done 59k . I can only just get to 55-60 mph . What model transfer box can improve this and can it be from a discovery.