I'm not sure what the point of the cap and PVC are. The tubing looks like tubing for an air pump; this would take a looooong time to siphon a gallon of water. I'd say just go to the hardware store and buy 6-7 feet of clear tubing about 1/2 to 3/4-inch in diameter. I'd also say to fill the hose at a faucet rather than suck on it, but that's just my preference.
The point as with all aquarium gravel filters is to reduce the suction by spreading the sucking power over a larger area so that the heavier particles such as sand have a chance to fall back down and onto the aquarium floor while the lighter particles like the usual dirt, bloodworms, axie poop etc. are sucked up and out the tube. Personally I'd want the cap to be a bit longer though, it seems a bit short.
The point of the PVC I think you will find is to make that portion of the tube rigid giving you more control without having to put your arm into the tank. You can hold the top of the PC pipe and still move the sucking end to exactly where you want it to go.
I do have to agree though that it would take a LONG time to do a water change with such a small hose.
You will find that if you attach a 1.25L soft drink bottle (or even a 600ml one) to the end of you 1/2 inch pipe and cut the bottom of leaving as much length to the bottle as possible you will be able to clean all the gunk out of your tank while removing very little of your sand you will even be able to push the bottle a little into the sand and back out again (this takes a little practice) to clean out any ditritis or other muck without losing sand. Removing more water faster as a bonus.
The larger/longer the bottle the less sand you will lose as the force is diffused even more. There is a limit to how big you can go with a 1/2 inch pipe though
Note Not sure how it would go with tahitian moonsand. I've never used it so have no real idea about it's weight compared to what you will want to be removing.
We have an actual Aquarium gravel vacuum (about 30cm long & 3 inches in diameter) that attaches to the garden hose and it works a treat and the water is drained straight out onto the garden.
No sucking of faucets (and risk of chlorinated water) needed either you can get it going by filling the bottle and holding it up a little so the water starts draining, quickly put it back in the tank to refill and repeat until it's sucking by itself and you're off and racing.