I can think of several ways, depending on your particular trunk configuration.
If you carry much in your trunk, then under the overhang area immediately behind your back seat may be a place where you can design/build a bracket/box to be mounted.
For several years I used to keep a fishing pole strapped under that area using velcro so that I'd always have it in the car and it wouldn't be damaged by other things I typically carried, or might carry.
Not that I would recommend velcro, mind you. A gun isn't a fishing pole, and it certainly wouldn't be as secure as I'd like.
You may also be able to fabricate something to mount on the interior side of one of the rear wheel wells. For example, some kind of metal/wooden box with a lid/door/panel on it which could be bolted to the side there. It would offer physical protection against other items you'd carry in your trunk, be easy to pack around, allow access to the spare tire well, and provide a modicum of security by keeping the gun "out of sight, out of mind" should anybody view inside the trunk, and some minor physical security by being in a bolted, lockable box.