Slash cut sonotube sub

...I used to use 2 sonotubes in my old car in the trunk. I strapped them down to keep from rolling around. I now have a hatchback so I couldn't use the tubes. So, I constructed a 1/2 MDF and 1/2 fiberglass box with a Parts Express closer out woofer. I used the fiberglass to mold to the shape of the car at the bottom and then to save weight. But since I put 4 gallons of fiberglass into it, I dont think that was the case. It fits like a glove and is approx 2.5 cubic foot with 2 4"dia 17" long ports. The total package tips the scale at 60 lbs. Its been in the back of my car for about 2 years now, but as a test I removed the sub. Because of the wheel base of the car, and the extra 60 lbs all the way at the rear and the see-saw effect, there was a noticeable difference in the way the car performed. It constantly felt weighted down in the rear and was harsh over bumps and felt unbalanced taking corners. After removing the sub, the car felt 100x better and I would like to keep it that way although I need my audio too. Soooooo, I looked for a light weight solution and here is what I have come up with.

I plan on using a 16" diameter sonotube, but it will have to be slash cut, both at the top and at the bottom. According to my calculations, I can get 1.5 cubic foot of air space, so I should still be able to use my woofer. This should save a significant amount of weight, although I wont have the cubic footage like before. My question is, usually a sonotube sub does not need bracing around its circumference. But since I am doing angled cuts at both the top and the bottom, this changes things. Should I brace the circumference now?

Also, any ideas on how to make the angled "ring" and "end cap" for the slash cut sonotube sub?
 
wow.... sound like WAY more trouble than its worth, since the endcap would need an infinite amount of changing angles to ideally match tine inside of the angled sonotube, not to mention cutting a perfect oval is a *****. i would look for alernative sub placings. WHat sub specifically is it? one of those beefy 12s or 15s?

stiffer springs?(know NOTHING about cars im warning you, but oddly i have an obsession with them)

MAYBE youcould use baltic birch or whatever to lower weight.
 
Well it is this subwoofer:
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=299-665

The woofer is awesome but it is 35 lbs by itself. Add the 30 lb box and it is heavy, weighs the car down. Stiffer springs is not the answer, and since Ive taken out the sub, I think I've gained 1 more mile/gallon for fuel economy.

I havent gotten around to making it yet, but I think it will work. I will oval cut the MDF to make the cap and then use a sander to fit the end cap as best as I can. Goodthing the sonotube is slightly compliant. I will try different woofers to see which one will produce the best result, while staying light weight. Weekend project here we come.