A little update on my work:
Finished filling and sanding for the most of it, I reckon it's more then enough for covering it with suede or anything alike. Only if we decide to spray paint it, more sanding will be required:
After giving it a guide coat of black spray paint and sanding to find the high and low spots (still lot to do)
http://i851.photobucket.com/albums/a...597_cr_800.jpg
Finished as it is, after 3x more filling and even more sanding:
http://i851.photobucket.com/albums/a...090600_800.jpg
http://i851.photobucket.com/albums/a...090601_800.jpg
http://i851.photobucket.com/albums/a...090604_800.jpg
http://i851.photobucket.com/albums/a...090605_800.jpg
After I definitely decided to go with 4'' port instead of 3'', my son cut out a bigger port hole, to allow for the 4'' pipe.
http://i851.photobucket.com/albums/a...090606_800.jpg
Of course, it means that we'll have to make new port flare, so I started making a new mold for it, as the simple one I used for the 3'' pipe won't work on 4''. It's still work in process but here are some pics of what I did so far:
Cut 5 rings. Top three will help hold the pipe upright, 4th helps save some filler, and the last one will serve as a turntable. Should work, I hope.
http://i851.photobucket.com/albums/a...090607_800.jpg
Forming the flare mold I realized that a beer bottle diameter has the perfect shape for the flare ;), so it helped me to form the filler. Still need to sand it smoooof, but here it is:
http://i851.photobucket.com/albums/a...090611_800.jpg
Please note that flare molding is not my idea, I read about it long time ago somewhere on the net, so I tried and found the page again. It has heaps of tips and tricks and the guy is doing some amazing stuff there. If you like, check it out here:
http://www.subwoofer-builder.com/port-flares.htm
Thanks for watching, all comments appreciated! I'll try to speed up a bit, as I've got only 4 weeks 'till the boy gets his driver's license and there's heaps more to do here, so stay tuned :)

