Art, thanks for explaining.
Jay, FYI Art pioneered RCABS
As to your questions, you're on the money. A developed, built up internal watertight compartment is expensive, and it's because you're paying for someone's time to develop and build it. Like many other things, you can do it yourself for far less money, but have to invest the time. You can build a house for less than you can buy one, but you'll need to be ready to (and able to ) do a bunch of work. Or, you pay someone else to do it for you. As a side note, I'm notoriously cheap, and build my own (hey, it's a hobby, right?)
Many tube style compartments use a sliding tray to mount the components, which attaches to an end cap.
RCABS works well, as does a snorkel, onboard gas, displacement pump, piston tanks, etc. Lots of different ways to skin the same cat.
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