Subwoofer at calibra???

HI Leutz! Have a Calibra for a long time and would like to put a Suwoofer (kiss) in the back! The problem with Calibra is that the trunk is very flat for a good subwoofer! Does one of you know a good box that fits in the Calibra back, so that I too lie on the hat rack clean? Thank you! P.S. Writes net I should remove the trunk