Small rooms need lots of absorption.
A easy cheap way to get a lot of absorption into a room is to adopt a standard commercial drop ceiling into your design. You could use standard ceiling tiles to fill in the grid, or better yet, choose a thicker mineral fiber based tile or even use a standard tiles and cover it on the top side with a layer of additional mineral fiber [dense insulation batts over standard tiles are a very effective way to add absorption at low cost with easy installation].
Also, if you have exposed ceiling joists, you can simply fill between the joists with mineral fiber and then "upholster" your ceiling. Consider the wisdom of fire treatment for any architectural upholstery.
Using the ceiling as a large absorption surface allows you to add a massive number of Sabines to the room without using up any floor space. A good start to any small room project.
Also, you can build a lightweight frame, fill it with medium density mineral fiber such as Owens Corning 703 [at least 4" thick for a broadband unit that will provide some benefit on lows as well as mids, and highs is a good idea in a small room] and the upholster with a breathable cloth. Here is an example:
Dave French's Ceiling Cloud
Finally you could use a modular panel. basically a SCA that you hang from the ceiling versus in the corner. See the SCA thread for details on building such units.
StudioTips Corner Absorber