Vent cover for exterior siding

Hi everyone, I'm new and setting up my two grow tents (first is too small, so I'll use for seedlings and clones) in my basement. One will have a 4" fan and the other 6". I found a Y connection and a 4-to-6 converter, but my issue is what exterior cover should I use? A dryer vent cover won't let enough air escape at one time, I believe like 12 CFM, which is obviously not enough for two fans exhausting out of it. So, what do I do to keep the rain from getting into the siding/house? Any vent cover suggestions?

This is the hole that was already here from an old wood stove we believe...

Is the siding where the hole is covering concrete or wood sheathing?

If just the sheathing, I'd use a sawzall to make the hole larger, and put in a 6 or 8" vent with auto-closing flap. Hold the sawzall tight right to the siding to prevent it from tearing away from the house.

We should be able to do that, the hole is actually already 4" and only sealed with clear caulk. I'm sure we can increase it. Can you rent that saw from Lowes?

Oh and I believe it's only wood behind the siding, but I'm going to check now...

So it looks like they have a 6" pipe with a 4" pipe inside? Definitely wood between the basement and siding, so that's good...