Personally I like the headache racks also like Shane's, but seeing as last night I have started rebuilding mine for the 3rd time, I have some advise. First, if you don't like the factory built mounts (which I didn't either for my toolbox) you will definitely need some sort of cross bar like square tube or the like with some kind of gusset. Reason being with out that it will want to shake back and forth and not be very solid at all (ask me how I know, my first one was an angle iron frame around the front of the box with the back rack uprights welded in the center, sorry no pics). Second, ALWAYS measure twice and cut once. Nothing is worse than cutting up good steel to have made a mistake. Also plan it out well, think of everything you want to put on it ect. Make a list. It will come back to haunt you if you don't, like I said, I'm rebuilding mine for the 3rd time because I didn't think it through good enough. Look at some other people's who have made them, factory built ones ect. Third, I recommend sandblasting it or getting it done. I am extremely happy I did and it isn't that bad for price either. Spray primer on the damn thing, it will save you headaches later on. I didn't and even had mine sandblasted and painted with rust paint and it had started surface rusting in a matter or 3-4 weeks. Also use good paint, again-very worth it later on. Lastly I recommend putting weather stripping on the bottom of the rails that sit on the box to keep it from rubbing and making noise and protect the paint.
Also now I see you from Ontario, so am I, if you are anywhere near London you are quite welcome to come look at mine to see how it's mounted ect or even for a visit or if you need a hand I don't mind helping.