Been a while since I had mine put in but mine is torroidal, in the spare wheel well and around 70l? I get a 55l fill from empty.
Lasts me 200-230 miles on a 3.0 V6.
I had to raise the floor a bit, and used polystyrene sheet to do that. Cut it to shape, laid it under the boot carpet and put that back on top. Made a carpet-wrapped fillet at the boot opening to hide the edge. No need for wooden frame. No problem with heavy objects as the load is well spread by the carpet assembly.
Works a treat, and only lost a small amount of boot height. Less than 2 inches, probably 40-50 mm or so? I can measure it if need be as it's been a while since I did it.
I don't notice the loss of space really, the fuel savings more than make up for it. Still managed to fit a dishwasher in there!
No spare wheel, but in 25 years of driving I've only had to change a wheel at the roadside twice, and I have recovery.