Infinity Kappa? Yummy... some of my favorite car audio speakers of all time.
How do it is fairly simple.
1. Remove your stock speaker boxes
2. Trace the shape of the box onto the MDF. Cut it out.
3. Mark the mounting screws for the stock box. Drill those holes out.
4. Trace the shape of your 6x9's in the middle of the cutout, along with the mounting screw holes.
5. Cut out the hole for the 6x9's, and drill out the screw holes.
6. Screw (nut & bolt?) the 6x9's to your MDF bracket.
7. Mount the bracket w/ speaker to your rear shelf.
This will make a free-air bracket. If you have subwoofers in your trunk it's advised to keep your rear speakers in a box to allow them to play in a separate airspace. Also test the excursion of your speakers to make sure it won't hit the bracket. The thickness of the MDF can act as a buffer to space out the speaker cone from hitting the rear deck under extreme use.
Here's what a store-bought adapter looks like: http://www.bavariansoundwerks.com/pr...I=445/_cid=247