I'm assuming in the last picture, you have the ass end of the battery sitting on top of the metal chunk that holds the 'foot' of the battery. If the battery tray is welded to the car, you could buy a universal battery tie down.
Like this, but the J-shaped bolts are sold separately with that one. You could drill 2 holes for the J bolts, and there you go. If it's not welded, get a universal battery tray.
Sitting at a slight angle is not a big deal. Not being held down is actually way more detrimental to a battery. I would be more concerned with the bottom of the battery sitting on the sharp metal edges of the battery tray for long periods of driving, it might cut into the bottom. Though they're usually pretty thick. If you wind up leaving it at a slight angle, I'd get a strip of wood, the thickness of the vertical walls of the battery tray, and maybe 3cm wide, just for the high side of the battery to sit on top of, so the sharp metal doesn't rub through the battery.
If you want a more redneck solution, get one of those little ratchet straps, hook one end to somewhere on the fender side, behind the battery, throw it over the top of the battery, hook the other end to somewhere below, and tighten it.
Is a 124R satisfactory for you? Or you still want more juice?
Like Bryan said, you could put the battery in the trunk... or you could keep the battery under the hood and ADD a battery in the trunk / rear / whatever. There are certain issues, but simply having a second battery should be fine. You can get a heavy gauge cable for the positive run to under the hood, which are a little costly. If you wanted to do it more properly, they have a switching device that will automatically switch the car's charging/load to either battery, depending on which needs it, while the car is running. Not sure what it's called.