It's been a while since there has been a post on this topic, but I thought my lawn chair modification to the i14t might be worth considering.
$11 dollars at the local hardware shop and two off cuts of 4B2 have provided a simple and comfortable solution.
I initially tried the seat without the 4B2 base, but seat frame rubbed onto the side tubes. Whilst this didn't seem to be doing damage, the 4B2 laid on the flat provided enough clearance to avoid rubbing. It also helps tension the seat. As shown in the photo, the standard seat is fitted over the bottom part of the lawn chair. The straps are then tightened. Sliding the 4B2 under the front and rear frame provides additional tension. It also elevates the seat and keeps my rear end dry! It also provide storage space for shoes and a drink bottle.
Although hidden underneath the bum pack, the top of the padded seat is secured in place by the two front straps which contact to a D shackle and then tightened.
I've tested this configuration twice, and found it easier on the legs and back.
Yet to see if it will also work in the forward seat or will interfere with the mast.
Note: The 4B2 is closer to 70x35mm or about 3"x1.5"