My husband built one for our kids with no plans. I don't think it was any cheaper than buying a kit though... he tends to overbuild relative to the kit and then justify these kinds of things by saying, "It cost the same, but I made it SO MUCH BETTER!" He's an engineer so he can't help it.
FWIW, I'm also an engineer but I much prefer the cost/benefit analysis model where it will cost the same, be less work (not take a month to do) and still be "good enough" that my kids will likely enjoy it about the same.