I would do your main part of your cage from the specified thickness required by SCCA, which I think is 1.5 x .095 (double check), and then I'd do all the additional supports in thin walled tube, like 1.5 x .065. It's not going to save a huge amount of weight, but it adds up over the whole project. If you decide to go with 1.625" though, paper towel rolls fit nicely over that size tube, so you can screw around with notches on cardboard and then trace them onto the tube once you have it the way you want it.
OK, some of this might not translate too well due to the angles, but hopefully it all makes sense. It's basically a very slight modification of Dan's entire setup with the hoop in the passenger compartment, an X to the front strut towers, and the bars from the bottom of the A pillar to the strut towers like my cage has. Also changed are the main hoop supports and bars from the door bar to the hoop; they now meet together in the front of the strut tower. There is a thick metal plate on the top of the strut tower. Weld your mounting plate to that and you should be good to go. Could even wrap up over the top or onto the wall a bit for a 3 dimensional mounting plate.
Here are my pictures of the bar from the A pillar to the strut tower. It really just barely fits and I had to locate the bar further away from the strut tower itself than I would have liked. The closer you get to the end of a tube the stiffer it is (you'll verify this if you get a tubing bender), so it's much better to be right next to the end than in the middle of a span, so it's still pretty good.
And here are my drawings on your pictures. I also am not a paint master, so hopefully you get the idea. One thing to take note of is the straight shoulder bar. If you are over about 5'8" tall (guessing) then you might have issues with the shoulder bar interfering with the seat going all the way back. I bent the shoulder bar to allow the seat to go all the way back, and that basically means that I cannot really do the bar from the door bar to the strut tower. Those two cannot coexist built off of the same node, note Dan doesn't have one either. Alternatively you could move the shoulder bar up or down or move the door bars up or down. Ideally you would want the diag to meet the hoop supports, but again, if you're tall you can't have that in a Z because it prevents the seat from moving as well, so that frees up the main hoop support location. I put my rear supports just under the bend in the main hoop, and then put a plate on top of the strut tower trapezoid and welded the support to that. If you decide to ditch the door bar to rear strut tower bar, then you can do this too, and the support would then basically serve the same function in a T bone accident. Might be higher than one would like though.
The one thing I didn't include from my own cage is the X from the rear strut towers to the bottom of the main hoop. It was a real pain in the ass to weld in, so unless you're hyper-anal like me, I'd suggest skipping it. If you do add that, cut the stupid shelf out first instead of trying to leave it in the car.
I'd be curious to see what some of the other guys have to say as well.