By far the friendliest and most generous Part 15 Rule is this one
§ 15.23 Home-built devices.
(a) Equipment authorization is not required for devices that are not
marketed, are not constructed from a kit, and are built in quantities
of five or less for personal use.
(b) It is recognized that the individual builder of home-built
equipment may not possess the means to perform the measurements for
determining compliance with the regulations. In this case, the builder
is expected to employ good engineering practices to meet the specified
technical standards to the greatest extent practicable. The provisions
of § 15.5 apply to this equipment.
The only member I know who has designed and built his own part 15 transmitter is Neil, Radio8Z, unless you count the committee of ALPB Members who designed the Big Talker shortwave transmitter.