Actually making progress over the last few days! Mounted the new tank first, then started making the seat pan.
Started off by cutting out the rough shape out of some .080 aluminum.
Started bending.
Rough shape finished, the bends still need to be perfected and the pan itself is still way too big.
Here you can see the super hi-tech tank mount (made from an old power steering bracket from my Miata that I happened to find on the floor

), and how I'm using the original front seat mount slot.
Front of the pan.
Covered up the tank mount.
And reinforced the front pan mount.
Tonight I picked up a yoga mat to use as padding (closed-cell neoprene foam), and finished shaping the pan. Then I played with the foam. 5 layers got me at the height I wanted aesthetically, and it puts me in a really comfortable position. Much better than with that 1" thick temporary seat I've been using.
Going to glue the foam together and shape it after work tomorrow, then hopefully go for a ride.