Here is another compact trainer that lets you work your core and move your pelvis in all planes. The OPTP Pelvic Rocker comes with a slip-resistant base and can be used in seated or supine positions.
This trainer activates your inner core muscles and promotes better muscle coordination. You can use this to perform pelvic tilts, pelvic clock, V-sit, and other moves. The OPTP Pelvic Rocker is available for under $40.