OK I get it now. You want to seal off a small grow space
inside the clothes closet. In fact you are thinking of a cabinet grow inside the closet. Is that right? Something like the picture of the grow cab you posted.
I had thought you were going to use the
whole closet and that perhaps, in the fullness of time, Hello Kitty would find her way to the Salvation Army store, or perhaps the animal shelter. So I was talking about closing up the main closet door.
In that case your idea of a plywood roof and door makes sense. You probably should attach framing timber to the wall and screw the plywood onto that. 2x2 will be fine for that.
If I understand your plan, the cab has to fit under the clothing rail. This gives you about 5' height. A fairly tight space. People use cabs this size but space is limited and you have to plan carefuly. In that picture, the top 12" or so of the cabinet is used for storage space. I would have used the extra height for the plants.
PS Here's a setup with a 600W in a small cab
http://https://www.rollitup.org/general-marijuana-growing/320402-stelthys-600w-hps-project-new-4.html
The lamp takes up about a 1'. You need about 10" of space under that. The pots are probably 9" tall. So, out of 60" height, you have 29" for the height of the plant. Probably less by the time you add the thickness of the plywood and the supports for the lamp. This will work but you may have to stay on top of the plant growth and do some training. Growing a tall sativa in such a cab will be a challenge.
If you hinge the door, will you have clearance for it to swing open? You might still consider curtains for the cab.
I think you definitely don't want a 400W in a cab that size. I'm sure people do it, but getting rid of the heat is going to be tough. If you make a mistake or dont catch a temp problem in time, a small space like that will heat up fast and cook your girls. You want them around 75F. If they get up to 90F they may go into stress mode and start producing male flowers and pollenate themselves in which case you will get a crop of schwag. The smaller the cab and the larger the lamp, the more difficult it is to manage the heat. My grow closet is about the same size as your walkin and I have some trouble managing the temp with a 600w HPS in there.
Do some searching on this forum to look at other people's cabs. What size lamp do they typicaly use? How much forced ventilation do they need? How do they manage the restricted grow height etc.
Some other ideas to consider:
Think about taking down the clothes rail, partition off the back of the closet floor to ceiling, cut the rail short and remount it on the partition wall. That will give you more height and more air space to work with.
You could also partition the back of the closet with light proof curtains which will solve the swinging door problem and require very little wood work.
