The TR-Plus is a transmit/receive switch with the added capability of feeding back some of the transmitted signal back to the transmitter. This allows the transmitter to adjust the signal to compensate for distortion, something like an active noise cancelling headset. This comes as a kit and is easily assembled.
I also designed and printed a simple case to hold the TR-Plus. My SMA jacks are not perfectly aligned so the holes had to be adjusted to compensate. Thus, they aren’t quite even.