Ive done a makeshift structure before and this is what Id recommend for cheapness yet durability(considering you dont drop kick it.)
Here is what you will need at minumum:
15 2x2x8's
2 2x2x12's
9 sheets of 4x8 paneling
Some hinges, weatherstripping, and Spray foam insulator
Assemble this struture(assuming you are building off 2 existing walls) and with some of the 2x2x8's make a header for support, and stick a panel on it with hinges and weatherstripping as a door. Spray foam insulation all around every gap from the outside.
Here is a suggestion from my build that I really love. Save a portion of the top of your structure and leave it framed but not finished. Buy a sizable box fan and fill the gap with it, blowing outwards. This creates a strong negative pressure that will keep your door closed and your grow clean.
If you need an extra set of hands Im free sometimes.