You can make a beer bong from basic things that they sell at any old hardware store. You can generally pick everything up to make a beer bong for under $15
You will need a funnel with a wide opening, 2 1/2 inch diameter tubing, clamps and optionally a ball valve.
Sometimes you can find a funnel that has a built in ball valve. These are optimal but if you have to assemble it yourself it is no big deal.
First, take your tubing and cut it to roughly three feet long. You can make it longer if you like, but the longer the hose, the more necessary a second person is to hold the funnel up high. If you cut it roughly three feet, you can still hold the funnel in the air on your own.
If you are using a ball valve, fasten the ball valve at the end of the tube, the other end of the tube should connect to the funnel. Use the clamps or some heavy duty tape to secure the tubing and valve in place.
Having a valve makes drinking the fluids significantly easier as you can flick the valve and start drinking, If you don't have a valve you will have to bend the tubing to prevent the liquids from coming out which will wear on the tubing over time.
The final step is customizing your beer bong. You can spray paint it camouflage or cover it in skate decals, whatever makes you happy!
The next step is to get your self a beer and test it out!