Heating a box (as a thermodynamics dummy)
Basically a thermodynamics problem with thermal conductivity, area, thickness, ΔT 

q=​d​​KA(T​.hot​​−T.​cold​​)​​  [watts]

Where:
q = Conduction heat transfer (W)
K = Materials thermal conductivity (W/mK)
A = Cross sectional area (m²)
T​.Hot​​ = Higher temperature (°C)
T.Cold​​ = Colder temperature (°C)
d = Material thickness (m)    


Convert your area 30x30x50cm =     6,600 cm² to  0.66 m²
Look up K for wood and choose d  
Compute Q heat in watts.


But if this is too slow, adding forced air evaporates faster  Then the air flow volume and rate has to be pre-heated and forced thru the box at the desired temperature.

This is more complicated by the efficiency at which air temp rises thru the heat per unit volume of air.  A radiator has high efficiency, a circular tube is lower coupling but allows higher flow rates.

I might suggest a 50W power resistor (s) mounted to a CPU heatsink ( tap and screw or clamp) from say an ATX PSU using 12V, using a CPU Fan with a variable speed control and same for heat control and temperature sensing to design a servo loop for the heat and fan speed. LM317's can also be mounted on heatsink for each so no heat is lost here.







simulate this circuit &ndash; Schematic created using CircuitLab

A Cheap and Dirty solution for fan control might be to use 5V on a 12V fan to reduce flow rate since evaporation rate is all day, as long as it is a good fan that starts at 4V.

For compact units choose an old Pentium heatsink. ( free at most repair stores)