it's the word "unobtrusive" that is limiting you.
want to hide your antenna? It's not going to work very well.
want a small invisible antenna? It's not going to work very well.
want to mount it low to the ground? It's not going to work very well.
HOWEVER! the nature of DX signals and the way the ionosphere bounces them around means that when skip conditions are favorable, you will hear stations coming in to your radio even with a sub-par antenna. you will even be able to talk to some of them sometimes.
so, you will still get the occasional DX, but you may find yourself frustrated trying to get them to come back to you due to all the other more powerful stations in your area who are also trying to talk to that same DX station.
a good base station antenna will be around 18-22 feet long, and should be mounted 36+ feet above the ground to the base of the antenna.
yes, it will work much better at 72 feet to the base of the antenna, and yes, it will still work if its only 18 feet to the base of the antenna.
If i had a big metal shed with a metal roof, i would buy a V 5/8 wave antenna, a 30 foot push up pole, and a three foot tripod.
I would mount the tripod right in the middle of the metal roof and use UV resistant guy ropes (made of dacron) to guy it in two places.
then i would run heavy gauge solid copper wire from the base of the push up pole out to each of the four corners of the shed, and down to an 8 foot ground rod for each wire run.
then i would run the heavy gauge wire around the base of the shed attaching all ground rods together, and tie that run in with the AC mains ground coming in to the shed.
Run the coax through a real lightning arrestor before it comes into the shed, and tie that lightning arrestor to your ground system.
yes, it's complicated, and can be expensive, but this should give you a guide for all the compromises you might have to make in your particular installation.
no matter what, you need to research about grounding and lightning protection.
hope this helps,
LC