Portable A/C unit in garage; Need help with the exhaust tubing

I would mount the vent tube attachment to the door, cut a hole and put mesh on it that’s the same color as the door. My portable ac has caps for when I disconnect it. But I run mine out a window.

10,000 btu portable will be underpowered for 400+ square feet. You're going to want to get a 14,000 btu.

Eta: you could also cut a hole in the ceiling and vent into the attic, or also consider getting a mini split.

Is your garage door and walls insulated?

I found that an insulated garage door made a huge difference in the temperature of the garage. If the walls or ceiling are not insulated, you won’t be doing much with that AC unit.

I wouldn’t use a portable. You should get a window unit if possible. You can mount a window unit to your exterior wall, to an exterior door, etc.

If you do get a portable, get the kind with two air hoses. They work much better.

I own multiple portable units and a window unit I use in my rental properties when an AC goes down. I don’t know the size but they aren’t small and the portables are great for a bedroom but much more than that you are really pushing your luck

I would have to agree with the mini split. Most models are heat pumps so you would have heat also. I have put in several of the ones off the internet. Have had really good luck with pioneer brand. Only problem I have found is getting parts for them compared to buying one local. I stopped installing ones off the internet for customers around Covid because of lead time on parts. This may be better now. Can’t go wrong with a Mitsubishi but they are a little more expensive.

One option we have done for a customer on a portable unit is to cut a 4” hole in the wall and put a dryer vent cover on it. Makes for a clean, ordinary look on the side of your house and the hose just hooks up like your dryer would when in use.

If I recall, a single hose portable actually is better than a dual hose when the outside air is extremely hot and humid compared to the indoor air. Basically in deep summer in south Louisiana a single hose is better. But anywhere with low humidity and on high humidity but sub-90's temperature outside and a dual hose is better.