I built one last summer using an old Tent Trailer frame. See post "'78 tent trailer to 2015 wheel house". Not sure how far you are along. I used 2x3 for frame, cutting 2x6 in half. I found 2x2 tend to split when nailed or screwed and if you have any size knot, the piece will break. Hard to find any straight 2x2's and even 2x3 that's why I used 2x6 and split them on my table saw. Glue every piece where wood touches wood with a good quality construction glue, don't go with the cheap stuff. Make sure it is exactally square and bolt or screw the corner studs together with 4 bolts/screws along with lots of glue.
On the floor, I'd go with 2ea. 1/2" plywood with 3/4 pink poly sandwiched between them. Bottom piece run plywood left to making sure seams are on cross members. Top run front to back. On steel use self taping sheet metal screws only pre-drill each hole - hard to get the screws threw 11ga steel.
Putting 2 benches in will take up a lot of space. I just use the fold up fabric chair with arm rest and backs. Very comfortable and moveable. They are hanging in the rear corner picture on my post.
