Welcome to Orchid Board
OK, so even
some firm root is good. (I have one with completely no root and that's going to be tricky, it was a gift but the person who gave it to me overwatered before I got it
).
Anyway, I would get a tiny tiny pot, maybe even an old food container such as a tiny yogurt pot. It needs to be the smallest you can possibly fit the roots into. Make sure it's got holes in the bottom, make some if you need to.
Now you could really do with fresh high quality bark to pot it in, but if you are fairly new to this you probably don't have any. You may be able to find orchid bark in your local shops, the problem is that it can often be already decomposed or not even the right stuff (something like Miracle Grow brand is too heavy a mix for Phals, from everything I've read here). There
are some good ones, but it needs to be light and airy with large-ish lumps of bark, not broken down like soil and you need to make sure it doesn't smell at all (really it should be sold bone dry).
www.repotme.com is meant to be good from what I've heard (I'm the wrong side of the Atlantic to use it).
If you are really stuck use the best, biggest pieces fom the old medium (from the top/drier pieces are also best).
The plant may be unstable in the tiny pot, but pot size should be based on roots, not leaf span. I use an outer cache pot to help balance ones like this.
Pot it up, and water regularly but try and ensure the bark is fully dry between watering. This is the reason for the small pot, it should dry quicker allowing you to water more often and that allows the plant to absorbe more water in my experience.
Also water with luke warm water. That combined with the coming spring should help trigger root growth.
I've saved a couple this way in the past... although my current rootless one is another matter
Good luck.