I've tried the vacuum adheasive stuff and as long as the bike is upright or you don't try to move it it works ok. However if your putting a tank bag on it with much more weight I would be a little wary of it. I picked up that magnetic square for like $20 and it does not come off at all, unless I want it to. Right now I'm just using a tank protector just for the part your family jewels rub up against and it's very firmly on there, not only does it have some vacuum action, but it's a magnetic solution as well. I just cut the piece into something that resembles a normal tank protector and you can't tell the difference from a "professional" one and this. I was going to buy one from these guys as my friend bought one and they are of good quality, but I just cheesed out and got his and traced it on the sheet I bought but if you want something more "professional" see the link below:
http://www.mag-knight.com/Tank%20Pad.htmThey also have sheets you can buy as well. Fortunately my bike is black so a black sheet works fine but if your bike is a funky colour maybe these guys would have your colour:
http://www.mag-knight.com/products.htmTheir sheets are 30 bucks but they have a funkier more professional surface to them. Mine is just vinyl black very much like the black mag knight protector. In fact you can't tell the difference between my friends "official" one and the one I made, except mine doesn't have the little logo on it.(Sorry mag-knight I'm a cheap bastard)
If you have a sheet on the top of the tank you can go totally hog wild and that tank bag is not even going to touch the paint. As a side benefit it will hold more securely as the vinyl surface has more grip than a fine gloss coat of paint. When you sell the bike you just peel the magnetic vinyl off there and the paint underneath will pristine like it came out oft the factory.
The one I made is a copy of this and look completely identical including the surface: