You could try and cut any type of fine mesh and maybe superglue it inside? I had a small internal filter with big slits and put a pair of tights on it lmao
Silicon a flat sponge. I did this for something similar in the past, it only failed because I fucked up the seal. Use CA glue and then silicone at the seal.
If the filter is removable, I’d get a sponge filter and air pump. You get one for $10 to $20. Well worth the investment. I run a sponge filter and airstone.
kaijujube 
i’d search etsy. i have the fluval spec with similar slots and there’s tons of sellers with 3d printed stuff
Velcro tape the filter and fashion a sponge or screen to cover the opening.
You could try and cut any type of fine mesh and maybe superglue it inside? I had a small internal filter with big slits and put a pair of tights on it lmao
I went on Etsy I think and found a person who 3D printed covers for Spec V filters. Might be able to find something there.
Why not just go with a sponge filter?
I have the same one and I have a little cutting of filter material I just have laying over the opening. The water pressure keeps it in place
Place a sponge or filter floss in front of it.
Silicon a flat sponge. I did this for something similar in the past, it only failed because I fucked up the seal. Use CA glue and then silicone at the seal.
If the filter is removable, I’d get a sponge filter and air pump. You get one for $10 to $20. Well worth the investment. I run a sponge filter and airstone.
Use pantyhose and cover the whole filter 🙂 make sure to find the most shear pair available near you