There isn’t a standard buckle, which means whatever you buy may approximately fit your car. This approximate fit might be fine, or might fail when a large force is applied. I wouldn’t.
It’s worse than that, you’ve got the extender tongue going to to the receiver, the proper tongue going into the receiver of the extender, and unknown quantity in the whole stitching and material of the extender itself.
Loads of failure points.
Plus, depending on the booster design, the new receiver may be resting on hard bits that you don’t want it to.
What was wedged may no longer be wedged because of this.
I’ve seen post crash photos (and videos) of the interiors of cars which started out with “well wedged” stuff in the boot.
I strap stuff down now. Only needs a luggage strap or a couple of decent bungee cords for light stuff.