Blast gates are leaky. I’d go with a louvered vent.
Like this:
Livtor Plastic Louvered 6 Inches Dryer Vent Cover Outdoor(Total Unit Size 8 Inches x 8 Inches) https://www.amazon.com/dp/B08B4QPWDC/ref=cm_sw_r_cp_api_glt_fabc_8RYRYX7TAX8BQ4JY75N0
On the absolute coldest nights you can leave the front door of your glowforge open, then your machine will stay room temp.
The louvers will prevent back flow. I’ve had a setup like this for 4 years and it’s great.