As above just use a couple of rubber stoppers with host clamps around them.
In my experience the worst offender for fuel leaks/fuel smell are senders & internal fuel pumps. Always seem to be the cause of fuel smell in the boot.
Here is a photo of the old blue Cobra's boot.
Would have liked to put the amp somewhere else, but we had stuffed full every nook and cranny. Was going to put a nice polished engraved cover over it, but never got around to it. That amp is bolted to back of a sub box. The sub box used up the entire area above the diff. We had blended in splits in the front and hidden head unit. Most people didn't realise the car had a stereo and that's the way we liked it.
Fake floor, to hide fuel tank etc.
There is also a full tool kit hidden in the side of that boot.
I can remember stuffing that boot full to the top on the many trips it did to Nationals etc.