boxer battery cables

That's because the posts on older batteries were of a larger diameter. What I did on my BB512i when I installed an Optima battery was to put a small piece of metal on the side of post itself before putting the cable on...it tightened up fine after that. Or, you could file a little of the material off the ends that go together on the clamp to make a smaller diameter clamp to fit on the smaller diameter posts on the battery.

 

Just a guess, but the end might not tighten on the post because the end of the end (the part where the bolt goes through) are coming together before it can tighten up.
Try taking the end off the battery post,
take the bolt out of the end,
spread the ends of the end a little, and with a handy suitable file, file some of the material out from between them.
Should be lots of 'meat' to work with here, so file out enough so that when you clamp the end back on the post, it clamps tightly before the end of the ends come together again.
Shouldn't take more than 5 minutes!

 

battery shouldn't die after 2 weeks of sitting, might be bad or have a drain from it on the car.

Ammeter gauge should show if alternator is charging or not, if you have a bad connection on your batt from the cable the ammeter will show this as its not charging fully or use a voltmeter at the battery to see if its getting 14+ volts when running.

You can check for a drain or short by disconnecting a batt cable and lightly touching it to a batt terminal if it sparks when everything is off you have a short somewhere probably the alternator, might also be a frayed batt cable.

If the cable ends next to the posts are real hard and brittle you might not be getting a good connection through them as well.

 

Could be a bad battery.
Might take it out and bring it to a Autozone or PepBoys they will load test it on their machine for free.

 

13.8-14.2 is about right, I'd put the new battery in and see what it does, the old may have had a short in it so the voltage reg didn't think it needed a charge.

 

Try this first: unbolt the neg battery terminal from the grounding point on frame, take a piece of emery cloth or steel wool and make the contact point of the cable-to-frame shiny new steel, do the same to the cable end, rebolt togather tightly and spray bolt and cable end with a little flat black to prevent future corrosion.
Did this on my TR, all of a sudden car would start quicker, run better, and the battery would not go flat. Seems like there's a marginal grounding there from the factory. I had to shine off the paint and undercoating to get a good ground.