I have been using a SkyAssistant variometer for a couple weeks now in a Radian Pro. It weighs less than one ounce and transmits climb/decent tones and voice alerts on the FRS frequency band (requires a $30 walky-talky for a receiver). It is sensitive to 2 ft/min altitude change. It is almost like cheating. The cost is $329, but if you want a real variometer, it is one of the options.
The only issue is I am working through some interference issues between the FRS (460 MHz) transmitter and the 2.4 GHz Spektrum AR6210 receiver. I think that I have the problem solved and will verify this with a couple more flying sessions. I can provide more details about my experience if you decide on this option.