Limit MPPT solar charger output voltage
A simplistic solution is to mount a large amount of 30V zener diodes in parallel.
They will swallow the over voltage and turn it into heat.

Google "DIODE ZENER 30V 5W" for 5Watt diodes. 10 of these would withstand 50 W continuous. There are larger ones, but they cost more. You will find them on ebay and the usual electronics distributors like mouser, farnell, digi-key among others.

This block would be connected across the poles of your charger, i.e. in parallel with your batteries.
Be cautious though, as they have a sloping "on" characteristic, i.e. they may waste a little current at normal operation. So maybe "31V Zener" is better, if available. Also, more diodes in parallel make for "quicker high consumption" i.e. less voltage rise before the combined pack starts to draw current and load your MPPT. I found the descriptive image below by googling. (Note these weird diodes are used in reverse, hence the "reverse" voltage and current).