Volume calc. of irregular sub enclosure

I am going to put a 15" driver (Memphis 15-PR154D) in the rear of my 2005 Suzuki Aerio SX.  I intend to fabricate a custom enclosure that is "molded" to the spare wheel space (the trunk floor) and is flat on top. 

The problem presented is that I need to be as accurate in the enclosure dimensions as possible to 60 litres.  I have not done any enclosures of this type before and wonder what the best way is to calculate that volume with a molded back.  My idea is to line the interior with a heavy-grade plastic, smooth out all the air so that the plastic is form fitting and then pour exactly 60 litres of water into the space.  I will then mark off the height of the water line (I know I have to consider the displacement of the driver and wires, but don't know exactly how to mock them in the water).  Would this give me an accurate template with which to frame my fibreglass enclosure?  Has anyone done this before? 

Does anyone have a suggestion on alternate methods that would be as effective and aesthetically tight?  I want a perfectly integrated driver with color-matched fibreglass (or maybe plexiglass?) top... 

Thanks for your input!

Volume of a cylinder = pi*radius(2)*height.

Use that to get a basic idea of volume for the cavity.  Basic because its not a perfect cylinder and you just need something to go off of.  Then take your mold and build your box.  When you are done, measure how close you got with packing peanuts and add or subtract volume as necessary.

Either I don't understand your response, or you completely missed my point.  The wheel well is an imperfect surface with about 24 different "folds" (peaks/crevices).  I want to use that as a back to my enclosure.  How would the volume of a cylinder work for me?  If I wanted to use a standard shape enclosure, I could simply "hide" it by fibreglassing the top.  That's not what I want.  I want an irregular enclosure to improve sound quality from the sub-bass frequencies (I've heard this makes a tremendous difference).  I'm a SQ guy, so - again - any help would be appreciated.

Thanks...

look at the thread entittled "STICKY PACKING PEANUT VOLUME MEASUREMNT" !

why you are measuring  in litres, I don't know(your signiture says your in the US).  But, the methood is simple.  Make a cardboard box that is 1 cubic foot (1ftx1ftx1ft).  Aquire some packing peanut, or even wads of notebook paper.  Fill the cardboadr box with the packing peanuts or paper. Dump it into the wheel well.  Each dump will be 1 cubic foot.

Yeah... I read those forums.  I can see that it works simply by the number of people who subscribe to the method.  It seems so inexact (Geek Alert!).  Since packing peanuts are porous and "collapsible", wouldn't the volume calculations be subject to just how energetic you're feeling that day?  I mean, two packing peanuts stacked end-to-end would yield a different volume than two stacked side-by-side and squished down, right? 

Anyway... I've modified the subs I'm using.  My original thought was to run a single Focal 25A1 (10") since I'm using Focal front stage and I'm very interested in voice-matching.  I won't be "bumping" - my intent is to get as near a perfectly transparent system as possible because I listen to a huge variety of music (Rap, Pop, Grunge, Ska, Classical, Bluegrass, Indie, Oldies, Rock - old school and new, Talk XM, Sports XM, etc...) and don't want a one-dimensional system as is so often found in cars today.  Since I decided to run my entire system off the Memphis Belle 600 5-channel amp, just putting 100 Watts on the 25A1 didn't seem feasible.  I then put my mind on that 15" Memphis driver because it just seemed to "fit" my plans.  I've scratched that because I realized I can run two 25A1 in parallel off the Belle 600 and get 200Watts using roughly the same space as the 15" Memphis (60 litres).  So...

I simply need the easiest way to figure as close as possible to 60 litres (2.12 cubic feet for you folk born here in America - I'm from the UK) which will give me an F3 of 38Hz - as deep and responsive as I want.  Perhaps I'm overthinking all this.  I usually do.  Thanks for the tips and keep 'em coming!

Charles' System:

JVC KD-AVX2 Head Unit, Rockford Fosgate 3Sixty.1 ISP, Memphis 16-MCH600 5-channel Amp, Focal 165A1 Front Stage, Focal 165CA1 Rear Fill, 2 Focal 25A1 Subwoofers (soon... currently running Blaupunkt GTA4 to 4 Polk dB650s off the factory head through the RF 3Sixty.1).

That's my dilemma - finding the rough volume of the spare tire well.  I think (after overthinking - lol) that I will guesstimate a 2.12 cubic foot enclosure and mock it up with MDF, then use the packing peanut technique that's been so well-described on this site.  Then, I can rebuild to my liking prior to actually forming the fibreglass enclosure.  Thanks for all the tips!

You can always use rice but there are microscopic cavities which aren't filled. Then again try Jello. Pour in liquid form then wait for it to congeal (remember to grease the trunk and put a stick in the middle for easier removal)