Hi peparami, that setup sounds like its gonna be a bit of an animal! (sorry could not resist
Quick question will you be selling your HK amp? My wifes one has a dead channel.
Re the subs etc are you doing a fully mounted set up or just lose boxes?
If you are going for the somewhat limited boot space I would line it with lots of layers of masking tape, then wet layup fibreglass (about three layers should give you strength without flex, you can add more if needed once you pop it out of the car), then once its set pop it out and build an mdf front and top on to the shell.
This will give you the maximum space/volume of air in the box, it is also worth putting wadding (remove guts of an old fibre pillow works) in the box as this in effect increases the speakers internal air volume (i know it seems weird filling the space to increase space)
Have fun and wear gloves, post lots of pics, and turn up the bass!
Ps let me know re the amp