Most of my attachment builds are due to financial limitations i.e. I can build it cheaper than I can buy it!!
That said, there are few that I expect to be improvements on what can be bought. If they work - Good!! If they work well - Great!!
Also, because of the limited size of my 4XX series machine, and it's limited capabilities, finding factory built attachments that are a direct fit and appropriately sized is difficult.
The plus side is I get some "Fabrication Therapy" out of it as well. :-D
As far as plans go, on some of my more complicated builds, especially ones where cylinder geometry needs to be correct, I do generally do some limited CAD layouts, and sometimes burn files for my buddy's laser cutter. I would hardly call them comprehensive plans that anyone could follow.
FWIW - SR