Yeah plywood is cheap and easy to build with. You dont really need it 100% sealed. Even if there are some air leakes, it wont matter as long as they are not very large. It will just be sucked some air in at the leaks aswell. As long as you got more air beeing sucked out then inn you will be fine odor wise. Light leaks you just cover up with some tape or something when its time. You can find the light leaks very easy once you got light turned on inside the cab.
Having both intake and outake on the same wall is possible, but then you need a fan inside of the cabinet blowing air away from the outake so that you create a cross current inside of the cabinet. Having them on oposite sides will give you more aircirculation for free. I do not have any experience with a active intake/intake fan. I only use fans on the exhaust and it works great.
Just try to get the air exhaust to suck the hot air away from the light, and most of the job is done
Seeing you got so little space to work with in height i would think it would be simplest to just hard mount the lights to the ceiling, and add some rails on the base so that you can pull the bottom out and work with the plant right outside of the box, and just push it back in when you are done. If you run soil you can just make the drain holes of the pot go trough the base connected to the rails, so you pull the base out, water the plants, have the run off water fall down in a small tray under the base and push the base back inn. You can also build a shallow DWC. It will probably save you some room and i found it to not be much more costly, and alot easier to wrap my head around. You also need to decide if you want to LST or ScrOG the plant. I found ScrOG to be very simple, have not tried LST, but it looks very simple aswell
Remember to do some research on DIY coal scrubbers for computer fans, as some computerfans simply dont got the static pressure to work with a coal scrubber. Would suck to buy a cheap computerfan, just to replace it later.