Well the point of a softbox is to make light less harsh -- to take your small point source of light and expand it for softer shadows. It's also to control the direction of incoming light so you can shape your subject in a controlled way. Sure you won't have room for a large device, but really anything could be better than a bare flash (depending on your intention).

Worst case, cover your colored walls with cheap white sheets or a white paper roll or whatever and treat it like a giant DIY light box.

Photography is all about overcoming limitations, or figuring out how to make it work just well enough to pull off a shot. Sometimes that's dealing with sub par gear; other times it's limited space or some other frustration.