first custom built box

What's up everybody, just ordered my 18" bl yesterday from Fi and now I'm designing a box for it (nothing special) But I just want to make sure it's gonna be done right before I start the build of course. Tell me if something is off here for it to be tuned to 31 hertZ... Height- 21" Width- 34" Depth- 21" For a total of 7.15cf And the slot port 3" wide by 20" height making the port length around 17" depth Can anybody tell me if it's not gonna work or anything but I've had my resources and this is what the specs for my build came to, thanks guys!

you need to take out the displacement of that sub woofer ad you need to add bracing inside a box that big so that it doesnt flex under heavy bass notes. also when you use the port calculator, the volume of the port doesnt count, so as it stands your port takes up another .73 cubic feet. take that .73 away from your original 7.15 and now you are at 6.42, this means that your port also needs to be longer to keep your 31Hz. after you play back and forth with the numbers i settle at a final airspace closer to 5.5ft^3 and a port length of 24 inches to maintain a 31Hz tune. if you need to have 7 cubes of air for that sub then you need to make the box's inner dimensions around 8.5 cubic feet to account for port, sub and bracing displacement.