The Tarahumara pack from Hill People Gear is a great daypack but some folks find it a little small. On my last outing, I needed to add a bit of capacity, so I strapped a MOLLE II Sustainment Pouch to the compression straps, to add ~430 cubic inches of carrying capacity.
This worked well for me but if you need a more secure solution, the MOLLE II Waist Pack is better because it will actually clip on, but is slightly smaller at 390 cubic inches.
Another option would be to attach the waist pack to the bottom of the Tara, with the paracord I used to strap my poncho to the bottom. This would allow you to use the waist pack's belt. You could then also attach the sustainment pouch to the back of the Tara for even more capacity.
My waist pack is in Multicam and was made by Propper; it's brand new. They can also be found in woodland, UCP, and desert camo patterns. I've been using a woodland one to hold a poncho liner thantI keep in the back of my truck. It fits the woobie perfectly.
Credit for this idea belongs to "Creaky Bones" at BCUSA. After taking the two pictures above and going back to Creaky's post, I noticed that he has his MOLLE II waist pack attached with the zipper towards the Tara, rather than on the outside. It might flop around less during strenuous activity if you do it his way, but on the other hand, you'd need to unclip it to access anything inside.
You may notice in today's pictures that instead of the bicycle water bottles in the wand pockets, I have Nalgene Oasis canteens, which are the same size and shape as USGI 1 quart canteens. They hold a bit more (32 oz. vs. 25 oz. for the bike bottles) and don't take up as much room inside the pack as the round bottles do. HPG specifically designed the wand pockets to accept the Nalgene Oasis and either these, or USGI canteens, are probably the best way to carry water in them. With two of these and two bike water bottles on the shoulder strap shock cords, you can carry almost 1 gallon of water with the Tarahumara pack.
Adding an additional pouch or two to the Tara potentially allows you to do an ultralight, warm weather overnighter, rather than relegating it to strictly daypack duty.