What ever signals you get (10 over, 20 over, whatever) have no bearing on efficiency, grounding, or anything else. It only means there is enough S+N/N ratio for you to carry on a conversation.
The most important aspect, is putting as much metal mass under the antenna as possible. Just remember, every HF installation is a compromise. It's how big a compromise you're willing to settle for that is the crux of the matter.
I admire your thoughtfulness about hiding the radios from prying eyes, and making sure the radios are fan cooled. However, I never recommend using boxes or other cramped areas, fan or no fan. While you'll more than likely get by with this, keeping them free and clear is also important attribute.
You might e-mail me some photos of the install, and I'll post them in my photo gallery.