All the one-container designs I've seen seem quite cramped although a 40x8 container is really a double or triple-length tiny house.
The best simple design I've seen is two 40 ft containers spaced about 6 or 8 ft apart (like your 30x40 concept I guess) and offset by about 8 feet lengthways - see the following pin:
Two Container House
This is really open and airy. Note the cutout section of steel from the side of one of the containers could be used for the slanting shed roof.
Another problem with containers is insulation - most people seem to end up building a studwall with insulation inside the container wall which really defeats the purpose.
One cheap option might be to bolt pallets to the outside of the container and then fill them either with insulation and cover with siding, or (my favorite) fill with soil and plant a vertical garden:
If you need more insulation put some sheet insulation between the pallet and the steel container wall.
The container roof can be insulated by putting a layer of straw bales on top and covering the whole with a pitched roof.
Another idea for hot summer - mild winter areas would be a roof pond or swimming pool (!) that would moderate the daily temperature swings.
Temptation is to use the pond to grow tilapia or catfish and filter the pond water through a grow bed on another part of the roof.