I've figured it out! Using available parts...
1. I didn't like the straight up sides and 45 degree roof because it was too tall.
2. I didn't like hoop supports because there was no top ridgepole connecting them and that was weak.
3. I didn't the gable design because it was too wide at the bottom.
4. I didn't like the tee design, because while it gave me a lower roof angle, the front an back were weird and it required way too many cuts and connectors.
So it FINALLY hit me like a cold wet fist in the dark of an alley (oh wait, that's a detective story). Seriously, I suddenly realized I could build a straight up frame to the height of the boat and THEN put in hoops on top into 45 degree elbows. I haven't seen THAT on any website. I'm sure someone has done that somewhere, but I haven't seen it anywhere.
So 1.25" pipe uprights on a sled base (no ground level cross-supports), 45 degree elbows on top with cross supports above the boat, and then 3/4" hoops over those. The 3/4" pipe will just stay in the 1.25" slip connections by torque! I'll use some solvent in there too. And my good idea is that the hoops won't just go from side to side unsupported, they will go into cross pieces ("Xs") as "half hoops" and the Xs will connect to a ridgepole!
That brings the height down to about 6', only 2' higher than the boat. And to give the top greater strength, I will attach plastic snow fence over it to support the poly tarp!
I have to rush off to sketch it on graph paper and list the parts and pipes I need! And its all available locally! I knew I would figure it out eventually...