Thanks guys
I'd probably not push it more than 75kg.. it may be up to more. It uses all the bolt holes on the rails.. so I guess it'll be up to what every the rails can officially cope with.
I've been prompted to build it as during our 2 week vacation we will be taking the dogs this year, so I have a large create filling the back of the pathfinder. So the rack will end up populated by bodyboards, camp chairs and windbreaks etc for the beach.
The ladder seactions are 20mm by 20mm box,
The tie down tubes front and rear are 2omm dia
and the sides are angled steel 30mm each leg, which came with slots already cut into it for bolting, so I only have to drill two holes each side and make a couple of slots wider as the rails curve.. I wanted to keep the bed flat rather than bowed.
The bed its self was some old galv mesh and was just the right size 148cm long and around 87cm wide. The gap between the rails was around 90.5 cm give or take around 0.3cm as the roof contored.
I welded the front and rear section then offered up the frame on some old seat cushions to protect the roof, then clamped in the centre cross section to make sure I got as close to the rails as I could.
The rest is time and paint

I also bought some plastic ends for the box tube.. to stop internal rot and also stop any howling from wind noise.. seems to have worked.
I've ordered a set of security screws to hopefully stop it doing a disappearing act.
