2 ways.
1) leave one plant space vacant or drill an access in the top tray
2) make sure the whole lid / top comes off.
If you look at that unit above, It is designed with with seperate Resevior.
Personally, I make access holes and use Plexi glass or a net pot lids to make covers.
PVC pipe and square fence posts totally waste your growing space. Plus it adds to the complication of having to have a seperate resevior with all the piping and stands that goes with it. If you have a 100% self contained unit with resevior and top together, you add that much more space to an already lacking space for plants. It will sit right on the floor and add that much more head room for the growing tops as well as space between HID lamps if needed.
Just imagibne that same unit above without the external piping. The pump sits inside the resevior and all piping is internal. NOTHING comes out of the res. (which also helps to prevent leaks, as well as moving the unit around should one need to. You can even place it on a $10 furniture dolly for mobility). If there is an external res and piping that becomes impossible). The plant roots hang over the sprayers and eventually hang all the way down into the Nutrient solution which is highly oxygenated due to the sprayers. Even if a sprayer goes bad, the plant still recieves it's water intake. A fail safe system.
Again, details, photos and full instructions will be here soon. I'll try and get some photos uploaded tonight so I can begin the "How To" this weekend. I'll do it in a journal style with steps. Step 1, step 2, step 3, ect.
Pirate Out.