Most of this I copied off of Boogers boat. I believe he said Crazy55 his brother built his rig. I made a few modifications to make it more solid for my big rear end.
Materials used; 43' of 2x2 steel sq tubing, 24" of 3/4 sq steel tubing, 36' of 2x1/8 steel bar, 24' of 2x1/4 steel bar, 1 3/4' bolt, 2 3/4" nuts and I bought
2 11' seat pedistals at Madisonville Marine for $11.95 each. BPS they are 19.95.
I cut the 2x2 steel tube into 1 37" piece and 2 3" pieces. My boat is 54" across where the front seat pin goes. I then drilled a hole in each end
1 1/2" from the ends all the way through the 3" and the the 37" pieces. Borrow a bit as they are close to $30. Weld a nut at the bottom of the 37" pice on each end. So the seat pedistals will go all the way thru and screw into the nuts. From there on you can see from the pics how it goes together. The bottom braces fasten to the 12' 2x14 steel bar pads.
You probably can improve on this rig but it works well for me. I bought the metal at a scrap yard for less than half of the box stores prices.
The 8" bolt will set you back about $8. I have $53 in the whole rig. If you cannot find the seat pins locally for less than $12 you can get them from Grizzlyjig.com.