I have built a QRP amplifier/exciter kit for the SDR-Cube.
This amplifier/filter is reasonably priced and works quit well. You can create your own low pass filters or purchase another filter kit.
I had a previously built kit that Steve Weber designed for his ATA series transceivers which contains a SWR/Power bridge and frequency counter. I incorporated this board, filter and amplifier in a SDR-Cube accessory enclosure to finish off the project.
My only complaint is that the Amp T/R relays are in the transmit position when powered off which means that if the amplifier is inline and must be power on in order to be connected to the antenna.
Here is a picture of my SDR cube, amplifier and Nue-PSK modem: