Here's a design that might give you some ideas... this one can be made very inexpensively from two Harbor Freight ATV Cargo Racks. Mine cost about $75 (the racks were on sale plus the 20% off coupon on each so they cost me $30 each), plus the $15 XJ Cherokee roof rack that's mounted on the roof.
It's actually a drill-and-bolt fab job, there's no welding or complicated construction or tools required. If you need more details or info let me know.
I've tested it with more weight than it would be safe to drive with, but that doesn't mean the factory hardtop would be capably of the same weight capacity, my hardtop has brackets that support the roof rack directly on the roll bars.