how to gather data of voltage and current produced by a solar panel when inside freezer
Assuming the light source is also in the freezer, and that you are not, then it is "easy enough" to bring out test points.

You need a current sense resistance to measure current - there are other means but that is easy and cheap. 

In normal operation R_Isense should be sized so that the voltage across Rsense is small compared to panel output voltage. eg if you have a 6V panel voltage under normal operating conditions then a peak voltage across R_Isense should be say no greater than about 5% of Vpanel = 0.3V, and ideally no more than about 1% = 0.06V. 

In this case if the load is a resistor for test purposes (which is a good idea) then both V and I can be determined from a single reading if the value of R is known. If the load is not resistive - eg an LED or light bulb or motor then a small sense resistor is needed.

Define:

Panel max power in test = Wmp
Panel voltage during test = Vmp
Panel max current in test = Imp

(1) For a resistive test load.

eg set load to load panel properly at max power.
R = V/I  = Vmp/Imp = 6V/0.1A = 60 Ohm load resistor.
Now Vpanel = V load resistor
I panel = Iload resistor
But I = V/R so Ipanel = Vpanel/60 in this case.

To measure Vpanel externally you need run only 2 wires out of the test area to an external meter.   From Vpanel you cam also calculate I panel as above.

(2) For a non linear test load - eg LED, bulb, motor etc.

You will need to use Rsense as above to measure I panel.
You will now need 3 wires from the test space.
Connect Vpanel+ to load+
Vpanel - to Rsense+
Rpanel - to "ground".
Rsense - to ground.
Run wires from 
- Load+ (= Vpanel+)
- Rsense+ (= Load-)
- Ground

Vpanel = Vpanel+ to ground voltage
Ipanel = (Vsense+ above ground) / Rsense. 

Set Rsense to about
R = Vmp/Imp/k
or 
R = Wmp / Impp^2 /k

where k is between say 10 and 100
(Lower affects Vload less but produces lower Vsense readings.)  

ie Rsense will be about 1% to 10% of the effective load resistance at full power.

___________________________________

Using a sense resistor - Example only

Vmp = 6V, Imp = 100 mA, Wmp = 600 mW.
k= 100 say

R = Vmp/Imp/k   = 6V/0.1A/100 = 0.6 Ohms.
or 
R = Wmp / Impp^2 /k = 0.6W / (0.1A)^2 / 100 = 0.6 Ohms (ie same result).

For ease of implementation use 1 1 Ohm sense resistor. 
Then at eg 100 mA, Vsense = IR =  0.1A x 1 Ohm  =  0.1V. 
ie Vsense in mV = I load in mA.

_______________________________

If Rload is inside the test chamber it will affect the temperature a little.
If the light source is inside the chamber it will affect the temperature - how much depends on intensity and efficiency of the source -
eg a 300W halogen bulb at about 2 feet is approximately as bright as sunlight overall but has far more % infrared than sunlight and will heat a test chamber substantially.
An LED source will provide less IR per light output BUT the narrow bands of light energy may cause lower net panel efficiency. 

__________________________________

Panel operated as an ~ current source for output measurement.

A good way to measure panel output is to operate it with a load resistor much lower than the optimum resistor that gives max power. It then operates as an approximate current source with Ipanel ~= proportional to light energy.

This means that Vpanel will be much lower than Vmp and not a useful indication of power output, but it is directly proportional to panel current and a good indicator of panel output under various conditions. 

"Shorting" the panel with an ammeter (which will usually have low resistance) is a good approximation to the above. 

A multimeter on a 200 mA range will usually have a resistance in the 1 to 20 Ohm range (some but not all are nearer the low end of that range.)  

A multimeter on eg a 10A DC range will usually have a resistance under 1 Ohm and is a good load for a panel acting in current-source mode. 
_______________________________________