There is also the
Goldtouch Go!2. There are a couple wireless versions, with bluetooth or dongle.
The Goldtouches are not strictly one-piece, but the hinge can be locked rigid.
BTW, there are wireless microcontroller board designs
on Github, which are pin-compatible with the Pro Micro, and which could therefore (with some effort) be used in many DIY keyboards ... But I don't know if you could buy them anywhere. I suppose there have been limited runs of them on local boards here and there.