I was just looking at t11 siding at lowes It in wood it looks to be .59" thick 4x8 and in the other primed stuff it lower like .390 thick. In my area wood/ply is cheaper than the primed ply .I think their both reverse board and baton looking.
I would stay the 3/4" on the floor if it pt then I would paint it. Maybe figure a gallon to paint the whole inside.
Scott
Nothing is ever easy. The T1-11 siding is rougher in appearance, but is more or less 5/8" plywood, with groves cut in it to resemble batten board siding, plus the edges are notched to overlap at the seams. But since this is 5/8" plywood, it is structural. The 3/8" smart siding stuff makes for good siding, but needs to be installed over something else.......like insulation board, OSB, plywood, etc. It is a good siding product, but is not structural like the T1-11 is. But in either case, the seams would run horizontal and not vertical, so I would not use either for this.
My thought was to build the Woods Mini from 1/2" plywood only and if decoration was needed, nail on some vertical battens to make it resemble batten board. You should be able to get each side from a single 4' x 8' sheet of 1/2 plywood, with scrap left over for the fronts and maybe to use for parts like nesting boxes, etc.
I'd also use a single sheet of 3/4" treated plywood for the floor........or perhaps stick with 1/2 plywood and paint it with Blackjack or use floor vinyl?
Also, if you make the thing 45" wide, that allows you to use a single sheet of plywood for both roof components, with 1.5" overhang on each side......enough for roof edging and some trim below.
As for framing material, they used to make a size of lumber called a 2" x 3", which is a 1.5" x 2.5" size. Not nearly as heavy or cumbersome as a 2 x 4, but stronger structurally than a 2 x 2. If you could find it, it would be ideal for this project. *
* Reminds me of a lot of nice food recipes that have simple ingredients, except for one...........that one will be something like "eye of newt" or "extract of powder"..........something nobody has ever heard of and is impossible to find.