I got an i-home thing which plugs into the headphone jack or output on the home dock. It was $20. Only downside is it runs on batteries. But I have used it off and on for 2 months on the original batteries. I think you can get an A/C adaptor for it.
Soloist is a very nice speaker that docks most Sirius radios with the universal connector. It will also dock most ipods. It also has two auxiliary inputs to connect even more stuff as well. It is light, thin, sounds good and has alarm clock functions. Definitely worth looking into.
Here is what I use: a cute set of portable speakers to my Stiletto listening equipment.
They are Very Small and portable. They have a built in rechargable battery. They fold up to a size smaller than the stiletto itself. The internal battery is supposed to last 7-10 hrs. The sound is a little tinney, but from a small size speaker I would not expect otherwise.
It's small enough, and allows portability. This will be a good alternative to my Boombox as far as portability. I can use this with my portable antenna cable too!
I got mine for about $20. I've seen them on ebay for less than that. They are an OEM version of the LG Portable Stereo Speakers MSP-100.
Before I converted the XM2GO box I used these all the time in Hotel rooms. It also is useful in the rental cars that not have an AUX input for the radio.
I just looked on ebay and I see there are lots to be had
Depends on just how portable you want. I love the JBL On Tour speakers. You can buy rechargeable batteries for it. IT will run about 8 hours on a charge. It gets pretty loud and the bass actually sounds decent. I've had these for about 2 years now. Doesn't matter what radio you have it will work great. Available in Black or White. You can find this setup for $50 via the Internet.