I ran some simulations and you're going to be just fine. Response changes only imperceptively. In fact, you've got a pretty large window of 30Hz and 10 cubic feet to 45Hz and 2.5 cubic feet where response is linear.So don't make any changes unless the port is too near the rear of the cabinet. As long as you've got a couple inches clearance, you'll have the same response with the dimensions you gave as with those specified in the plans.