I have a 944 with an aftermarket system that I installed and during that install I overcame just the problem you are having with your amp. I also have just purchased a 968 and plan an identical setup with it.
In my 944, my amp is mounted in the same cubby space (left side) behind the wheel well, but is secured in place and I did not have to drill into the car in order to mount it. I'll go take a picture and post it if you want, but hopefully this description will get you going.
The amp is screwed into a piece of 1/2" thick plywood. The plywood is cut to fit vertically in the cubby and is used to "hang" the amp on the inside surface of the cubby area, with metal brackets bent over the small metal ridge at the top of the inboard edge of the cubby. All of this sits flush under the carpeting and no drilling or screws are needed. I've been running my 944 for over 10 years in this configuration with absolutely no problems.
The mount to hang the plywood is very simple. The bottom side of the plywood (opposite the amp) has a couple of thin rectangular metal plates screwed into it, the ends of which slightly overhang the edge of the plywood and I have bent over in a "U" to form hooks that the whole assembly hangs from. The metal plates I have used are from the hardware store and are traditionally used to nail two 2x4's together. They are thin, full of holes, and easily bent into whatever shape you need.
Some amps have settings that you may need to access from below, and I have correspondingly cut openings in the plywood to allow one to get at them without having to remove the amp.
You'll find that you'll need to use a jigsaw to cut the plywood into the right shape so that it both has enough area to hold the amp as well as still fit down in the cubby.
I did mount a cooling fan on my assembly, but have determined that it was unnecessary. You should be just fine with the amp as is because there is plenty of space in there.
Any questions, post them to me and I'll elaborate and/or take some pictures.
