Yes, the easy way would be to mount the amp to the sub box and just remove the both when at the track...but suppose one wanted the weight savings all the time.
Deck: Some deck with MP3 capability and a built-in crossover to eke out some extension on the low end. The CDA-9853 could work:
It has a "6-Position HP/LP Digital Crossover" and "50W MAX x 4 High Power Amplifier"...though we all know how worthless a deck's power rating is
Speakers: Probably the Cadence ZX-681
I've had good luck with this line in the past and the published sensitivity of 93 dB (yes, obviously inaccurate) is a good place to start.
If I felt like being more creative, I would experiment with something like a line array of these:
The Tang Band W3-871S, though with that impedance I'd need either two (not enough) or eight (maybe too much) to get the right impedance

. I've made multiple home speakers for fun using these. I'm not sure how well they'd do free-air though...maybe build a small enclosure for them...