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Screw an elbow on the spigot's back-end threads. The height of your drilled hole should be such that the the free end of the elbow is close to the jar's bottom surface. Works great on my 500 gallon water tank.
Very nice idea! I really like your oscillating saw idea! Once the panel is removed, one could pretty easily route/chisel/etc the arch at the top of the door into a rectangle. Rectangular glass will be less expensive than glass with curves cut in it. There would be fewer worries about losing control of the router in that large area vs removing the panel without marring the panel front.