a simple and effective way to do all of this is to make a floor structure. if you have access to MDF, foil, posterboard, a jigsaw, and screws, you will be fine:
step1 -- use aluminum foil to make a mold of your trunk. the foil can be shaped to fit the trunk. this should be as close as possible for best results.
step2 -- transfer shape to a piece of posterboard (large white paper). you will need multiple pieces. this will allow you to test the shape and confirm it fits.
step3 -- divide the posterboard mold in half (driver side / passenger side). each half will be cut from MDF.
step4 -- transfer each half to MDF and cut. place pieces in car. note any wiggle room. wiggle room is ok. (oh, and if you wanted your spare, you should leave a cutout for that!)
step5 -- these halves will be secured to each other by connecting a piece of wood overtop.
step6 -- place the amp/subs on flat pieces that will secure to this false frame.
and you can be creative from there. its a simple way to partially secure your setup. adding carpeting or spacers can remove any wiggle. you can also build a 2 level amp rack so the amps can sit under a layer of MDF, allowing you to place items above the amps to reclaim storage space.