I started out with a canister that I purchased a Walmart, a water tank, 1/4 id x 3/8 OD tubing, and four aluminum barbed tube fittings tube 1/4 id x 1/4 - 28 male pipe Mcmaster Carr part no 5058k34. I chose these fittings because I wanted to drill as small a hole as possible thru the tank.
using a 7/32 drill and a 1/4 -28 tap I drilled and tapped the holes for the fittings in the water chamber and reservoir. I also put some silicon sealant on the threads to insure no leaks. One fitting above the water line in the chamber and reservoir, one as close to the bottom of the chamber and reservoir as possible without hitting the bottom.
I then attached the hose, allowing enough hose length for the reservoir to sit behind the cpap machine on the nightstand. I tested this setup, but even though the canister was designed to hold a vacuum, it would not hold pressure.
I needed a canister that would hold pressure, but also that the wall thickness of the canister was thick enough to provide for a few threads for the fittings. I purchased a new one gallon gas can and installed the fittings, included a sight for water level, and used some 3/8 Herbie clamps to insure the tubing stayed on the fittings. It might not look like much, but it holds 4 2/3 cups of water vs the 1 1/3 cups of just the tank by itself, a little over three times the capacity of the stock chamber.
Keep in mind that this set up operates on gravity feed, you never want to let the water level go above the fill line in the water chamber. I also will eventually remove the site for water level since the water level in the tank does not go past the height of the T fittings installed in the lower tubing line.
The design was not perfect, but it sure works well. If anyone decides to build on, and finds a better canister for the reservoir please let me know.
