I like this idea:
https://www.speedsterowners.com/forum/readmsg.asp?t=10854
BUT, being a former car stereo nut, I would do it a little differently.
The tough part would be finding an old suitcase the right size. I used to find stuff like this at thrift stores all the time. What you want is the biggest one that will look OK strapped to your rear seat. A picnic basket would be ideal since the open weave would muffle the sound less.
Next just make an enclosure that fits inside snugly, out of 3/4" MDF. It doesn't have to be pretty, just solid and airtight. Ideally for this application you should divide it into two separate chambers, one for each speaker.
Now as far as choosing your speakers, I used to be a real snob about it but car speakers have come a long way. Sound quality in an open car is kind of an oxymoron, and car stereo buffs usually scoff at high frequency speakers placed behind the driver, but you are just going to want something that sounds clean at a high volume. You should be able to find some nice 6 1/2" coaxials in the $75-$100 range.
You are also going to need an amplifier. A four channel will give you more options down the road, also look for an adjustable built in crossover. You may never want to add a subwoofer but you can still use a high-pass crossover to dial out distortion. Don't worry too much about the manufacturer's specs, just go for a quality brand. Craigslist is a good bet here as the high end models are pretty durable and you can save a ton. Brand new expect to spend at least $100 for a mid range model on sale.
Head unit- here is where you will save some money, don't use one. An Ipod works great as a combination head unit and remote, and is a cinch to hook up to an amp. I picked up a 6 foot 3.5mm stereo headphone jack to dual RCA wire at the Shack for like 10 bucks. Even better than an Ipod would be a PDA with nav capabilities or a cell phone with an MP3 player. I picked up a 4GB SD card for $15, that's about 1000 songs with no CDs to mess with. Odds are good that you already own something that will work, just run the wire to a convenient location. Right now mine runs under the carpet and pops out under the e-brake boot.
Wiring- Amps are easy in a Speedster since the battery is right there and the "firewall" is fiberglass. Just use nice fat wire for power and ground, wire yourself some kind of switch for the turn-on lead, and run the speaker wires to the rear seat. You can probably find some fancy connector at the Shack to use as a quick disconnect between the box and the speaker leads.
This is going to sound a LOT better than a plastic box, especially if you find some good high speakers. It also has the potential to look a lot better in your interior, draw less attention to itself and cost less. Now for those of us with front speakers already, just substitute the component speakers for a nice 10" sub with the proper airspace requirements....
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