Ok, just got my fanny pack/murse.
http://www.timbuk2.com/tb2/products/pork-chop-belt-pack/2114828
Yeah, I know what I'm getting myself into. But I really support Timbuk2 (all their products are durable) and this particular pack allows me to actually carry my 3ds xl around. And there's still plenty of other space for other things if desired (phone, keys, wallet, etc.).
I wear it like a fanny pack on my right side fixed with my belt. The belt I use is a basic military style Columbia brand belt (fairly sturdy as well).
http://www.amazon.com/Columbia-Cotton-Washed-Webbing-Black/dp/B001FOR2CY/ref=sr_1_1?ie=UTF8&qid=1376606222&sr=8-1&keywords=columbia+belt
It's basically like a glorified Camera belt pouch. However it lays flat and doesn't bulge out. I also like the look of it being square and not having those tapered belt ends regular fanny packs have.
If I'm paranoid about the 3ds getting damaged or want to carry more games, I also use the AmazonBasics case in tandem. Still fits with plenty of room.
http://www.amazon.com/AmazonBasics-Carrying-Case-Nintendo-3DS-Lite/dp/B0050SVPCO/ref=sr_1_1?ie=UTF8&qid=1376606515&sr=8-1&keywords=amazonbasics+3ds+xl+case
I would recommend that as well since it has straps to secure the 3ds xl.
With this setup. I really feel like I can make use of the 3ds xl street/spotpass and pedometer functions.
Now, I have cargo pants/shorts so why not just put my 3ds in there? If you've ever put something with at least some weight into a cargo pocket you'll know what I'm talking about. Your thigh constantly swings back and forth. And there's nothing to secure said item in the cargo pocket, so it constantly fumbles around. Also, if I put something in their, I would most certainly forget about it. Most extreme situation would be that my 3ds xl goes in the washer, but that'd most likely not happen.
Alright, for the tl;dr here are the pros and cons of the Timbuk2 Pork Chop Belt Pouch
Pros:
-Plenty of room to fit 3ds xl (case too if desired) and other items
-Can be used with any belt (sturdier, the better).
-Attaches to hip (less movement)
-Lays flat, doesn't bulge out.
-Very durable, and items stay secure.
Cons:
-Glorified fanny pack/murse.
-$25+ (not that I think it's worth it, but this isn't priced to CAG standards).
Notes: No matter how you look at it, using a fanny pack is a fashion faux pas. In my case, I purchased the black one. IMO it stands out less than the other colors. The best positioning for this I found was on the exact side of your hip. If you wear in the front or back, it'll get in the way, when you either have to sit down in a chair, reach for your pant pockets, or need to take off the belt (bathroom, hanky panky).
One thing I have to test is permeability. I don't know how well streetpass would work when covered by the pouch (i doubt it'll be that much different from being in your pocket) and the pouch/case combo.