You might loose the rest. Push it back in and put a little Black Mastic or Clear Silicon under the last area before pushing home. Make sure the area is clean and dry. Wipe any surplus with White Spirit.
I gather he has paid by Paypal, send one last message giving a deadline, then refund and relist. Let Ebay know so you don't get two lots of listing fees.
I have a custom built sub box with a Rockford Fosgate sub fitted in it, that fits the whole aperture behind the original Bose cover. Imported this from the US. Pm if interested.