Hmm. Sonic Impact amp runs on batteries or wall-wart, about $35. Could be placed in the box if necessary, though access to the volume knob would have to be figured out. Need RCA to Headphone Connector - probably $10 by the time you buy the stuff and solder it up. Full range Tang Band 4" $20 (x2). Materials for box $20. So $85. Probably better off with a computer speaker system (2.1). I've got one from 5 years ago that was $50, includes both RCA and headphone input.
Is a single box a must.
Another option is a Soundmatters Mainstage. About $160 refurbished. Great sound for the size. Has two analog and two digital inputs (will decode DD). If you want to spend $150 on materials, this is probably a better option. It has pretty satisfying bass for near field listening.