I don't have a lot of experience with sewing, so I might just cut the holes and do a slip fit. I will probably do some duct tape with a hose clamp.

Honestly, I don't like the thought of cutting the tent, but I don't know how to get around this problem. One 12" inline pulls as much air as two 8". I might as well do a 12" intake and a 12" outtake. My only downside is that my hvac filter is a 8" with just one hole. https://www.amazon.com/dp/B092114P97?psc=1&ref=ppx_yo2ov_dt_b_product_details Do you think I could do a damper to make it fit? Something like this https://www.amazon.com/Duct-Reducer-Ductwork-Heating-Duct-Air-Duct-Ventilation-Fittings/dp/B00B79OH0C ?