I was shooting a video of how I did the seat, seeing as I'm not a very intelligent person, it did not go well. I guess you need to free up memory on your phone once in awhile.
I shaped the seat with the MFT 3, 1400 router, 5/8" Otto cutting bit, both wings from the UG stand on the right side/rail side of the MFT 3.
The seat is 488 mm in width.
Starting at the right edge of the seat, set the bit to zero. Slide the seat to the right 5/8", remove .5 mm. Slide remove .5 mm, continue until you get to the center. Now you have to raise the bit .5 mm with each 5/8" move.
The reason for concaved first is when you flip the seat, it now rides on the left and right edges. If you do the convex side first, the seat will rock.
To do the convexed side start on the right, set bit to 10 mm, route, slide seat 5/8", raise bit .5 mm, repeat until you reach the center. Now you have lower the bit bit .5 mm each move.
A few things to note;
The "rear" UG stand fence must be aligned to the MFT fence, both stands must be on plane with the MFT.
You need to have a pice of material the same thickness of the seat to support the rail as you slide the seat to the right..
There will be a minimal amount of sanding to blend the "steps".
It took me longer to type this than shape the seat. I'm sure I screwed up part of the process.
Tom