sounds like you got a sweet hookup on the rubber! (or plastic in this case
Right now I run one dry box and one cooler with no drop floor. IMHO putting a seat on top of the dry box just makes it too high for the oarlocks so I only run a seat if I don't have the drybox in there. just ethafoam on the box and either stack items behind me for a backrest for the mild stuff or a stadium soft chair or whatever they are called work fine too. seats are really subjective, some can kind of mess with the angle of your thighs with the oars and people are really divided on seat backs and whether or not they mess with your stoke, etc. you honestly just have to play around.
if you want to run a seat and the other stuff get the bighorn II and with the seat bar and the extra 10" of frame. then you have a lot more room to play around. I recently ordered longer side rails so I could add a drop bag and table to my setup but I'm still going to have to sit on my box. what I used to do is put a folding everlast table on floor in the back and stack everything on top of there. and that works good. water jugs go well between the foot bar and the cooler. I'm hoping the drop bag is going to help with some of the harder to pack stuff.