I can honestly say that the ostomy support belt from C M Ostomy Supply does hold the bag in an out-of-the-way position that you may find favorable. For one, you don't have to tuck the bag, which can possibly cause some leakage should you have one of those moments. While I have had a couple of skin barrier failures while wearing the wrap (but not because of it), it did a lot in keeping the mess to a minimum as opposed to not having it there to hold things in place. I almost never leave the house without it on because it feels much better than having a bag against your skin under your clothing.
I don't work for C M, but I recommend this support 'belt' for the aforementioned reasons. Fifty dollars is a lot to pay (in my opinion), and should you have any issues with the belt, the company will work with resolving them.
As far as not showing under your clothing, it depends on what you are wearing, I guess. I wear my shirts loosely, so people haven't a clue. Of course, if you are wearing something close to skin tight, you can see the outline of the belt, as well as the 'lump' the bag makes in the inner sleeve. Overall, it does a nice job of concealment, and surely in the area of making you feel most comfortable.
As far as the more intimate activities, it looks like an ace bandage wrap. It would be nice if they made more decorative belts. But it does wonders in keeping things tucked away during those moments.