I have used this to build my DL-650 air filters with DAP's Aquarium Sealant. (Yes, there will be an article forthcoming for doing this!). You can purchase it at Lowe's, or Home Depot. There are other brands as well, that I have used.
Shop DAP 2.8 Oz. Clear Silicone Specialty Caulk at Lowes.com
See this stuff that I am holding in my hand? Also, the package of Locktite brand behind, on my work bench. They work very well too.
You could prototype your own box using simple materials.
Kind of like how I make my own air filters. I have two OEM air filter "frames".
I cut the old filter medium off of it.
Squirt some glue around the base of the $5.00 filter.
And, stick it to the frame.
I've been doing this since 2009. So, roughly for 60,000+ miles. I just made two more up for this season.
Okay, where were we?
Oh yeah, waterproof, FLEXIBLE (because of expansion and contraction due to changing temperatures.) glue. Check out what I listed above.
There is also this. But, I have never tried it. Aquarium Plumbing Adhesive: Mr. Sticky's Underwater Glue