you'd still use three sheets for the 5ft so might as well go for 6ft and patch then sides together. I've done it a few times on the back of vivs when I can't get the full sized piece in the car.
you can use aquarium sealant to stop any moisture getting in the gaps, but with a beardie viv there shouldn't really be any.
something like this - Ronseal Interior & Exterior Woodstain Teak, 5010214833900 ; 5010214833825
just check the exterior varnishes and find the low VOC Ronseal one, and choose a colour.