Solar Panel + Controller + Diode to float LiPos
Not that I have tired. but if I am in your shoes.. I would get a DC to DC stepdown and calibrate it to 5V then use a 5V to LiPo charge circuit to do the job.. All these you can get easily off the shelf in plug and play boards.


The DC to DC board has the ability to deal with fluctuating DC input. Using LM2956 chips.. 
The 5V to LiPo charge board will depend on how big a current you can get off your solar panel.. I know of some TP4056 chips that can take 1A load..


This was is my intended setup I intend to use for a 2Phase (stepper motor) generator for my bicycle..   

Boards are from Aliexpress and very cheap.. so blowing them up will not be to big an issue.  Hope it work for you..