Harman P43 DIY OAK Question

Harman P43 owner here, installed it last winter and absolutely loved it! At the time, I didn't install an OAK (just a normal duravent thimble) but am now looking to fix that. I'm not buying an OAK kit, they're overpriced for what you get and like I said, the plain duravent is already installed and hooked up.

In looking at the OAK kits you buy, it appears the thimble is exactly the same except there's an extra 3" hole in the corner where the flex pipe goes through. My plan is to take a 3" hole saw, cut a hole clear through the top left corner of my existing "plain" thimble, then buy some heavy duty flex pipe. Seems very straight forward. Any reason why this wouldn't work?

So to sum it up, this is my current thimble: http://amzn.to/1umCHzL and I want to make it like this: http://amzn.to/1qjMmqW by cutting a 3" hole in mine. Any problems I'm overlooking?

Thanks for the reply Spock. I must have linked the wrong thimble because I just measured mine and it's 12x12, not 9". Here's an actual shot of the install to give you an idea of what I'm working with. I know I could just pop another hole through the wall next to the thimble and route the OAK that way but it's pretty tight between the thimble and wall and I'm leery about the room (keep in mind, there's a stud there too).

Can anyone who bought the OAK kit confirm that the thimble is much wider or is it similar to mine?

One more question for you guys: anything wrong with using a 3" stove intake pipe? I noticed that most guys use a reducer off the stove intake, then run a 2" flex pipe through the wall. I found that a 3" flex pipe fits nicely over the stove intake and I'd like to run that straight out the wall. If anything, I'd rather have more airflow rather than restrict it. Any known problems with running an air intake that large?