The output stage uses an inexpensive ($20) RA03M8894M Mitsubishi "brick" amplifier module. This is a MOSFET power amplifier, simple to use because there is no matching required; it's input and output are already internally matched to 50 ohms, so I just fed those with short .085 hardline jumpers to the SMA connectors. The other two connections just need a little bypassing.
The output level is controlled by varying the bias voltage. I mounted this in a small Hammond die-cast box, and the PC board you see was made by cutting connection pads with a dremel tool. The capacitors used here are SMT components.
Since it's bandwidth is limited, the module also works as an additional filter to improve the suppression of out-of-band harmonics from the 100 mHz diode multiplier.