well here are two scenarios. the first is what i did, the second is what i would do....
My platform was made with 1"x1" tubing with the brackets I started with an 13 1/2" long tube and put the brackets 1 1/4" from each end leaving 11" between them. The XOP brackets use 1/4" hardware and the Muddy Brackets use 5/16, so plan ahead for that.
I drilled a hole for the the seat in the center of the square 8 /14" above the bottom bracket and a second hole in the center of the square 3/8" from the bottom of the stick. The reason for the second hole is to store the seat for walking in an out.
I pressed a piece of 1/2" steel tube 1.60" long with a 1/4" through hole in the seat hole to take up the slack between the seat an the square tube. This eliminated the washers, and helped spread the load of the seat pin across a larger amount of the square tube.
I added a 1/2" thick spacer plate to the front of the tube to allow the leveling screw to line up correctly and spread that load across a larger area.
I decided to use a locking pin instead of a bolt, because when the seat is in the position for use, it doesn't fold up very well. by adding a second hole and using a pin, I can move the seat to the lower hole and then it folds up nicer.
Now if i were to do it again, I would make some changes. I wish i could find a 1 1/4 by 1 1/2 tube, but i haven't found it yet.
the main reason, is I would like to get rid of the 1/2" spacer. it causes the entire thing to fold up funny.
Using the 1 1/2" tubing, I would put the brackets in the same place but instead of putting the seat holes 1/2" from the front, I would put them 1" back for the actual seat location, and 1/2" back for the storage/walking location. This would allow the seat to fold up nicely for walking and yet still be in the right spot for use. If i was concerned about the tubing collapsing due to the leveling bolt, I could always cut a piece of oak with a tight fit and hammer it down the tubing until it was underneath the contact area of the leveling bolt.
Alternatively, using a 1 1/4" x 1 1/4" tubing, you could put the seat bolt 3/4" back instead of the 1/4" listed above and just get a slightly longer leveling screw. I would still put the stowing hole at 1/2"
Does that help?