I’m using Siraya nFEP film now with the stock Mono X vat and love it.

https://www.amazon.com/gp/aw/d/B09MRGRF4M?psc=1&ref=ppx_pop_mob_b_asin_title

That is what I used, it worked just fine with a water bottle cap in the middle for slack.

Just trim it once you have the frame clamped down.

IMHO, these should come with an extra fep and extra carbon element.

Also, I just picked up the siraya tech nfep sheets for my mono x and wow are they amazing. https://www.amazon.com/dp/B09MRGRF4M/ref=cm_sw_r_apan_i_ST0G1G2F1M77TYJ98ZRQ?_encoding=UTF8&psc=1