The vent exists for a reason. You cannot block it.

Extend the ducting so the vent exits the side of the bench.

Are you talking about a register that delivers warm air, or an air return?

Well...what does the vent vent?

I'm tempted to agree with your idea of closing off the vent...and to remind you to keep up payments on your house insurance and your life insurance...

But I won't.

It’s an air register that blows out air.

I was considering closing it off with a magnetic air vent cover.

Here is a link the the air cover. https://www.amazon.com/Deflecto-Magnetic-Cover-Sidewall-MVCX512/dp/B00009W3G4/ref=asc_df_B00009W3G4/?tag=hyprod-20&linkCode=df0&hvadid=167120909983&hvpos=1o2&hvnetw=g&hvrand=2201942865566059274&hvpone=&hvptwo=&hvqmt=&hvdev=m&hvdvcmdl=&hvlocint=&hvlocphy=9006954&hvtargid=pla-307523368555&psc=1

I close off furnace registers all the time. Many floor registers have built-in dampers to adjust or cut off the air flow. There is no safety issue. Ignore all the gloom and doom silliness and do what you need to.

It sounds like a heating supply duct rather than a vent. You should be clear what it is.

It is a register that blows out cold or warm air depending on the thermostat setting.

That is a supply register... not a vent.

Proper usage is important for proper communication and understanding.

Yes, it can be closed off if you don't need the heated and or cooled air it supplies.

It's still a good idea to keep your payments current on your insurance...

Okay, great. Thanks for the help!

Why not just leave it there if the bench isn't going to be enclosed? If it is, you could install a register at the front of the bench and it would still serve as a supply. Better than closing it off. Despite the "Doom and Gloom" comment, it's not a good idea to restrict airflow in your system by closing off vents completely - it adds static pressure and can increase air velocity at the other vents, sometimes making noise an issue.

Ok thanks everyone. I spoke to an HVAC professional and he said I could also find the damper to the register to close it. He said it is not recommended and would prefer to move the register outside the enclosed bench.