Next I traced the cardboard template onto the seat pan material. I used 3mm aluminium sheet, 'cause that's what I had hanging around. Steel would be fine. Cut it out at a mate's house on his bandsaw. Drilled a series of holes around the circumference of the pan, 5mm holes about 3/4" in from the edge and about 1" apart. These are for pop rivets. If I did it again, I'd drill the holes a bit further from the edge, maybe 1" or so. Luckily for me, same mate had a roller at his work, so he took it in and rolled a curve into it. You could do it over a pipe or log or something. He rolled it far too much, and into a kind of even curve, so some tweaking over a log to straighten the front and reduce the curve at the back soon had it looking right, feeling good and fitting the curve of the frame. No pictures of the bare seat pan.

next up I made the brackets. The frame mounted part is the only welding I needed to do. Just cut a peice of 1" x 3mm flat bar the same length as the rear tank mount and drilled holes in same places. Then, bent another piece of same flat bar into a square sided "U" shape with an inside width of about 1". the "U" is welded to the flat piece and a hole drilled through both upright bits.

Drilled a hole in the front of the bracket to match the one on the tank mount and slid a pin in there. the pin just floats and never fall out. It's like a hinge.

The seat fits with the pin at the front and the back just sits on the two frame horns the rear fender used to mount from. Some springs back there would be nice but I just put some heater hose on the horns.

It's not completely perfect, but I'm really happy. It's comfortable, looks cool and cost $0. What more do you want?  

Hope that helps and as always, my 2c, YMMV etc.
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Nice post.   I appreciate.  I will definitely give it a try.

Ya know,, some stuff "anneals" differently than ya'd think,, If you go pounding on copper, it gets hard and brittle, to soften it up, heat it and quench it,, If ya let it slowly cool, it stays hard. Ive never tried aluminum,, Once annealed, how is it UN annealed?
I got instructions from a columnist in a Homesteading magazine, ( mita been Grit) on how to temper metal to make a good punch or chisel. Its not SUPPosed to be so hard as to be brittle to the point of risking it chipping and sending tiny bits flying, but hard enough to cut into something like a black pipe dutchman busted off in a fitting.. It requires a product sold by a multilevel marketing bunch,, Its on a piece of cardboard taped to the wall in the shop.. And IF you ever need to heat metal to a target temperature and youve no way to gauge the temp of the metal, go to a welding supply house. They have "crayons" that change color at certain temps. Theyre spensive, BUT,, IF thats what ya need,, Like if youre wanting to weld cast irone, ya gotta have rods that contain Nickel and ya gotta pre-heat the area to a set range,, I was gonna TRY to save a vise,, It cost $26.00,, The rods and the crayon cost $20.00,, so, I bought a new one..

You have a rose-bud tip for the torch? You use acetylene or propane? I use propane,, its a little slower BUT Ohh BOY is it cheaper.. Takes a different cutting tip.. The brazing tips work okay,, but I wonder if theyd be easier to work with if they had acetylene.. But, a bottle of O2 mite last me a year, so,, Im not gonna spend the $$$ to find out, I dont do enough cutting and brazing usually for it to matter. I have a Smith medium sized torch and a small OHHH, dang,, its the most common maker,, senior moment,, sorry, torch.. I got a rosebud because I bent a wheel up REAL bad on the backhoe, I was running from a fire. I WAS fighting the fire, rolled over a Burned to a point stump, ruined the tire and hadda get away, so, the rim was bent all to crap.. IIRC I went thru 2 bottles of O2 in 4 days as I heated and beat it back to functionality.. Its tubeless and holds air just fine,, Beating on it was like hammering on a huge bell.. No dogs would get close.. I poked wires in the ground for gauges,, and used my experienced eyeballs,, Youd never know by looking what a disaster it was.. I TRIED to find a wheel,, but after 2 or 3 days of calling junk yards I gave up and just fixed the dang thing..

Dave, I forgot to mention the quenching part. Thanks. I was taught the soap way by my tech teacher in high school. I remember the project I made using that technique was turning a flat Ali plate into a soup ladle, just beating it round with a mallet over one of those stock things you stick into a bench. Can't remember the name of those things. He taught back then that the part will re-harden in a few days. Still have that ladle somewhere.
JOG, I don't know what a rosebud is. I got the welding and cutting torches and a great big multi heater. Uses too much gas and that's very expensive here. Have a little o2/propane set as well. It's good for brazing and silver solder. Sounds like the wheel was an epic project,nice work  

Just had a quick google: annealing ali, you don't quench it, let it cool slowly. Anyway, makes it nice and malleable.