I use styrofoam insulation -1". It's pretty cheap. Cut it a little oversize - 1/8" - and push it up between the 2X4's. It stays by itself. It is a smoke hazard if you have a fire, so keep your wiring legit. Don't plug 27 things into the same cord.
I'm against windows. It's hard to control the amount of heat from the sun in the summer and you lose alot of heat in the winter. I use heater vents. They can be opened and closed as you need and spaced where you want. Cut a hole and screw them right to the plywood. Another good way to vent that is even cheaper is to cut part of the floor out, cover it with hardware cloth and window screen, then nail some 1x2 slats over it so you don't step through and bust your ankle, or place the whole in the floor under some shelving where no one steps. I have a bathroom exhaust fan screwed to the ceiling to draw air in. I don't run that very often, only when it's really hot.
I use a sterlite box (16 Qt?) and an automatic watering system. It's hung from eyebolts that are screwed to the 2X4s for the roof.
I sloped the roof and the floor. If something spills it just drains out the side. To make the door, I nailed the front on, drew a door, cut it 95%, put on three hinges, and finished the cut. Bingo, door. You won't find it in Better Homes and Garden, but it works. You can screw a 1X3 on the side opposite the hinges. Bingo, door stop.
Good Luck,
DeMak
P. S. it's cheaper if you draw it out first. You won't get everything perfect, but your sure to eliminate a couple of screw-ups.