Doors as in how you enter the room ? Just use a regular door just line the inside with sticky foam strips or any or door jams that seal the bottem you ca. Usually buy them in long strips simple effective and cheap
I assume by box it's not very big and you can't walk in it. I made a grow box for my first grow. I got some particle board from home depot and had them cut it to spec. I drew out the specs on a piece of paper before doing so. You should do the same. Then i bought some hinges from hobby lobby and put it all together.