modified Mini scooper 18'' on a bike

I want to improve the sound of my bike. I really need to get a better bass going - the roll was mainly historical as on the old smaller bike a scoop would not have been possible. I would really like to have a 18'' neodymium driver. And I found this plan here:  www.freespeakerplans.com/plans/14-plans/...30-super-scooper-18q
It already looks really nice - but unfortunately it is a tiny bit too large. The bed of my bike has the dimensions of 59cm x 65cm and it needs to fit in there as modifying the frame would be difficult. I would be really happy about ideas. Ideally it should look like a small (dub)soundsystem scoop and give strong and low frequencies. (btw. Is there some software that can analyze my music collection and give me hints about the ideal specs of the drivers? I think I really want neodymium (for power and weight) and 4Ohms (as I cannot double up here). Would be really happy about Ideas and some URLs where I can read up on things.
I found this one that is really close to my dimensional constraints: 

 - only 1cm too wide - but unfortunately no plans.
I think this one is the best I have seen so far from the fit: 

 - unfortunately only a YT video and not a CAD file or a plan. If anyone has a plan for such a speaker (especially the clean inside) - that would make me really happy!
Driver wise currently the eighteensound 18NLW9601 4Ohm looks best to me

from the plan it looks like the b&c sub 18 design, or like. 

see plan in this post.

Thanks for the suggestion!

Seems this one has some more depth than I can provide. Wondering If I keep the volume the same (making it more wide and high) and adjust the size of the port in the same proportions if this would work. Sorry for my ignorance here - so much to learn

You can do that, what matters is that the internal volume and the (frontal) port surface area and port depth/ length stay the same.

You're a really inventive chap. There is a lot to be said for tubes, by the way - they don't distort with the sound wave like a flat panel will (this is the design idea of Dave Rat's bass speakers). If you don't know of Dave Rat, he is worith looking up on utube. I am wondering what you use that bike for - are you a modern day troubadour?