Considering the shown test fixture and some later ideas I have, here are some more considerations that may help:
- About soldering: I think that with or without soldering, the fully plastic 2ct leds will have no long term stability, the plastic will shift, change properties and so on, and affect the results, both on short or long term.
My suggestion is to search for a batch of older glass lenses leds, US is practically flooded with hi-quality military and aerospace surplus and something like this should be available at less ferocious prices:
https://www.ebay.de/itm/1-Stuck-1N6266-General-Electric-GAS-Infrarot-LED-A10-2255/221670551615 - If you want an absolute low cost, use a socket, don't solder anything on the led pins, just make a socket from a good quality DIP8 with machined pins and adapt it to your fixture, this way you can test led with minimal thermal and mechanical stress and quickly replace them.
One other, a bit out of the box, solution is to use a power led on a ceramic substrate, that will be less prone to thermal stress due to soldering and the whole package should be more stable.
Like these:
https://www.ebay.de/itm/10-Stuck-1W-940NM-Infrarot-IR-LED-1-Watt/173357756157About temperature, there is no way to ignore it, either you ovenize the LED or put a good thermistor on it and do a calibration table.
Cheers,
DC1MC