There is no reason to put the minder right where the battery mounts. Any of the mountings above will be fine.
Now,
You have a problem.
Your battery may be marginal.
Your car may have an electrical problem. A current drain caused by some faulty componen
t.
You need to fix this problem. Your car should be able to start after sitting for at least 6 weeks and actually much longer.
If you use the battery minder to mask the problem, you will take the chance of having the car not start when you take it on a weekend trip where the battery minder is not in use.
There's nothing wrong with a battery minder. You shouldn't need one but have one if you like.
Just don't use it to mask a problem.