you can get a
bulk head fitting from your big box hardware stores or hydro store. basically its a threaded male and female adapter on one end with a rubber gasket. the hardware stores around me only sell a 1 1/2" fitting for $12.50. the hydro stores sell a drain and fill kit for around the same price. the ones i found are barbed ends for hoses. you get 1 bulk head fitting per kit. when you need 4 in one project both options where to expensive for me.
i tried hoses with clamps and also silicone on fittings and in my experience both leak. i tried to make my own bulk head fittings out of PVC fitting and they leak even with a gasket. then i realized PVC pipe has a tapered threads. you could wrench them on with a 24" wrench and they will never seal.
but electrical conduit fittings do not have tapered threads. any hardware store carries conduit fittings. they also sell gasket material. for about $1.50 you can make a water tight bulk head fitting that will fit any size pvc pipe.