Are you adverse to making something yourself ? I see you are familiar with Atmega and that has a pwm and adc capability. If you start small you can learn as you go, you already have a ~12V 50W panel just make your own pwm controller and start with that.
I could recommend an IRFB4227 mosfet as being virtually indestructible driven by a TC4427, also an MBR20200CT schottky is likewise almost indestructible those components can be re-used again and again and are good for up to 200V. In your situation you also need a decent film capacitor to absorb the ripple current, just invest in some Panasonic brown jobs (they have a proper published specification unlike so many others) a few 2.2uF/250V will do you. Just use your electric fire as a load and away you go for a simple PWM! As mentioned before it will radiate like mad without an inductor but perfectly ok for experimentation.
Initially the control loop could simply measure panel voltage and increment/decrement duty cycle to try and keep it at the specified (for the panel) MPPV. 25Khz is a good pwm frequency to go for at the beginning, not to much in the way of switching losses (or other parasitics) but high enough to keep component sizes small.
Get yourself a copy of LTspice, it's a really useful tool as is an oscilloscope if you don't have one already.
PS please dont use your SLA as a power source it has no current limit and is unsuitable, stick to PV