One thing i was wondering about with a homemade harness is if you can use more ply-able (sp?) webbing, like a seatbelt style, the regular webbing is kind of stiff after use and if dry (loosens when wet, but sometimes its hard to get it soaked when others are in the water etc), that stuff is just as strong even if its not as thick. I am not sure what webbing is used on parachute harnesses, its been a little while (8 years) since i did that - kind of forget over time if you dont think about it, but is that any more ply-able?
As for making the homemade, i can understand if your webbing wears thin and you still have the hardware to just make a new one with new webbing only for $4-5, but to make a new one when you dont know how it goes together could be harder and i really dont see much saving ($-wise and certainly not time-wise) in it compared to buying a ready made cheaper one like the DR.