i'll try to get you some idea's and measurements from our box stands, we pre-assemble them and can erect a stand at our fields in less than an hour, we use post hole diggers set the main post, x-brace and then take the sub-assem, starting with the floor and start screwing the 6 peices (the floor, the sides (4) one with the door in it) then the roof) together...

on your deminsions i'd go for 4x8 standard sheet sizes, ours are 4X4 and are a decent size for an adult and a younger teen, the 4x8 should be plenty for 3 adults..

seating, buy some of the fold out chairs that pleople take to ball games, they are reasonbley comfortable,,pad the floor with old carpet, i don't remember our window dimensions but i'm thinking 6"-8" tall all the way around...

i'll take some picks this W/E if you'd like to see

the price on ours runs around 150 a stand, all pressure treated stuff and metal roofs