I'm trying to figure out the correct size for intake and exhaust sizes for a 17x21x38" smoker. There will be a box under the unit for a AMNPS (mailbox size or smaller) I was planning on using 3 separate 3/4 holes on that on its front side near the bottom. Then using a 2" pipe about 4inches long straight out the top of it into the bottom of the smoker. Using a 1500 watt Brinkmann element, then a 2" pipe out the back of the smoker near the top. Will 2" pipe work or should I go 3"?