Just a thought (and will probably get many 'that looks horrible' comments) but I found a second-hand roof rack from a Mitsubishi Pajero on our local version of ebay, and bolted it to some Halfords roof bars - total outlay about £80. The roof rack and bars are aluminium, so don't rust and have no powder coating, and I can strap long timbers etc down the side of the rack (which is very useful for stacking them against) whilst using the rack itself for bales of hay etc (will take five comfortably).
Second-hand roof racks like this seem to come up relatively frequently. It was the work of a few minutes to chop out the side bars where the cross-member supports were and bolt onto the bars.