Slick!
When you drill into the black plastic be careful about hitting the edge of the glass. Those look like metal latches; won't they rust when salt water gets onto them? Most stainless steel will rust in salt water, and galvanized steel would definitely rust. Could you use acrylic latches?
Is there any kind of barrier (screen) keeping an octopus from going against the water flow and climbing up into the HOB filter(s)? I've never used HOB gear, but I would be worried about it. Does anybody have experience with HOB device security?