So for my Burning Man bike I've put on a tall (~8') bamboo pole with lights and such on it. I've used pipe clamps to hold it onto a reguarl rear rack, which work OK, and then last year a 1/2 vertical split PVC section with pipe clamps attached to a section of 2x8 which was held to my bike rack using U-Bolts. It worked OK, but put a LOT of horizontal stress on my bike rack, and it actually broke the rack right above that side's attachment to the frame. So I've been looking around to try and find a solid rack which I could be pretty sure would stand up to the stresses. Realistically probably decent steel, rather than aluminum. I have found some steel racks around (such as thetouringstore.com/thetouringstore/tubus-bicycle-racks-for-touring-and-commuting/tubus-cargo-evo-rear-bicycle-rack/) although naturally they're more expensive (up to astronomical). I'm still uncertain if they'll be really solid for the side-to-side and front-to-back stresses that having something that relatively heavy, and especially that far away from the actual attachment point on the rack will be.

I've thought of working up a mounting which clamps onto the top of the rack and holds it right at the end of the rack with the bottom just behind the rear wheel fender, although that will make the bike rather rear-heavy but I can deal with that, and stresses will be a bit more vertical that way. But possibly even more so side-to-side. Might be usable though.

Alternatively, I've considered making my own rear-rack, possibly attached in multiple points by P-clamps. www.bicyclinglife.com/HowTo/HeavyDutyRacks.htm has an interesting, very solid design. I can get to a local hobbyist metal/welding shop and so can cut/weld/etc potentially. Unfortunately that person doesn't give a fully detailed plans, but I think it's straightforward enough. That said, I'm not sure how side-to-side might be. Given it's stainless steel bar stock probably plenty strong, and stiff enough to not get out of alignment really.

Thoughts? Just go with something like the Tubus and it should be fine?

Tubus is made to take panniers. Sounds like your load is completely different. Really, a heavy arch with a horizontal connection to the saddle to keep it upright would probably work best.