I like your diy mindset! If for any reason you decide it’s not going to work, oil based thermostat-controlled space heaters aren’t too expensive. You’d need to totally redesign the enclosure of course but it would probably be a bit more “set and forget”.
From last year:
Combined with something like this…
Granted it’s closer to $200 than $30, but you’re talking about protecting a multi thousand dollar Glowforge, I’m not sure the absolute cheapest solution should be the goal?
EDIT:
Interesting, Wirecutter changed their tune and recommend smaller heaters now. The type I use and swear by is a 1500 watt delinghi like this:
DeLonghi TRD40615E Full Room Radiant Heater https://www.amazon.com/dp/B00G96S4Y8/ref=cm_sw_r_cp_api_xLCYBb182M7RK
The key features for me are the digital thermostat and timer. I set mine up to heat the space to room temp during working hours and much lower overnight. It works well for me because my workspace is pretty small, I think you’d definitely want an enclosure in a garage.