Well, based on the internal volume of the 1040, how about something like the SpecOps Brand X-6? I usually preach the road of non tactical, but in black it looks like a camera/binocular case, and the "speed back" system is probably the best version of the PALS/MOLLE system I've ever seen. You can put it on just about any kind of belt or strap, and with a pair of d-rings, a chunk of 1" webbing, and some triglides you can rig a sling in a few seconds that looks like it belongs. (In a hurry, skip the D-rings and triglides, just use a length of line or webbing.)

7.5 high, 6 wide, 3 deep, it will hold a pair of small Nalgenes (or a rectangular one liter), or a USGI canteen, so it should fit a 1L bladder pretty well. Or your kit as is, in the 1040. You could get by with the smaller X-4, but it would be borderline on the bladder (and being narrower, it looks a little more like an ammo pouch). The top is held down with both velcro (there is a silencer flap if you want to use it) and an side release buckle, and extends down a ways over the sides so you don't have a risk of loosing stuff out the sides. And they are just about bomb proof- it is probably the best buy in nylon I've found. They should run you about 25$ plus shipping. I've got a bunch of them in different colors and vintages, they are one of my "standard" packages.

http://www.specopsbrand.com/ProductDetails.asp?ProductID=46