Some questions first.
Do you want to leave the TV there all the time?
Is security an issue?
How big is the TV?
Any other components like speakers and stereo etc?
Do you want this to be permanent or removable with no change to the siding?
If I was doing it I would make it removable in such a way that it could later be removed, and the siding would look as if it had never been there.
Here's how.
I would unzip one piece of the siding where the top of the box would be. I would cut off the hook at the top of that piece in the area where the box would go, so that I could slide a flat piece of steel, used as a hanging bracket, up behind the siding.
Fasten that steel bracket through the sheathing, and into studs.
This bracket should have a bend in it so it will clear the surface of the siding pieces.
Over top of this bracket, and attached to it, I would install a piece of metal flashing wider than the box, that comes out further from the wall than the box does. This will shed water and keep it out of the wall and off the box.
The box, built as a waterproof construction, would hang from this bracket, clear of the siding. Done this way, you don't have to stairstep the back as the example above shows. You don't need to caulk around it either. You might add a pad to the back of the box near the bottom, where it rests against the siding.
Make the bracket substantial, so the box can't just be ripped off the wall. or as an alternative, make a similar bracket, installed the same way, near the bottom of the box. This one should not be as wide as the upper one.
Then the box is just a lightweight , fully sealed assembly with 2 doors that swing out to the right and left You could mount speakers on them. Gasket or weather-seal the box. Run your coax and electrical lines from up under the siding, where the holes won't show, into the box and seal with the seal they use on electric meter boxes. Securely lock the box and camouflage it's purpose by painting industrial grey and hooking a piece of PVC conduit to the bottom, running down to the ground. Letter the front with "High Voltage".