Have you discovered the DIY forum? If not it's at the top of the toking tools forum. But, a 5 gallon bucket gravity bong with multiple hookah hoses on it is always a great one! Great for those college house parties
While ago when I was bored I would take an empty 500mL water bottle, shove my 18mm diffused downstem into the top, then put a hollowed out e-cig near the top for a mouthpiece. Not really homemade since there is $80 worth of glass on it but it was fun to make and hit surprisingly well.
In college I made a bong using two smart water bottles and used straws (the one that bend) for the perc. came out pretty cool!!