The subject of DIY ROPs comes up regularly, and when it does someone always will point out that you can't DIY a ROPs, and that if you did, you could be in worse trouble with your insurance company in the event of an accident than it you didn't have one at all.
So I was amused to stumble across some professionally engineered plans for one from the US gov CD. Apparently when ROPs first started catching on some of the extension and gov agencies were hard at work on trying to come up with a cheap and cost effective versions for old tractors, before the spoilsports started screaming LIABILITY!
All bolted, no welds. Klutzy but about the only way you could get around the varying quality of "farmer" welds. But even if someone wanted to weld one it's a useful source of info on what size steel to use, as one assumes they did all the calculations rather than going buy guess and ROT.
Don't thing I'd make one. My state has a 70% subsidy to promote ROPS; the end cost to me would be about the same as the steel would cost for a DIY version.