ive found that the best way to do it is it to get a plastic trashcan, brute and tuffman make some good strong one, and hook the RO unit up next to it and just fill it up. the are easy to modify so you can fit a float shut off valve on them and prevent over flows. then i just purchase a lenght of tubing, how ever long you need it, and get a pretty powerful sump pump in the bottom of it.
its hard to explain but i make a connection with PVC to the pump and the PVC only comes out of the water a foot or so, then thats were the connection to the hose comes into play with a nipple fitting. i would take pics for you but i dont have a camera (sry) after you get this part done then just have some way to move the water around to keep it mixed. i find that a couple of small powerheads work fine.
there are many otherways and for me this works out great, even when draining the mix container down to nothing. i do appologize for no pics or anything, but i hope i explained this as good as i could for you lol. when it boils down to crunch time, anything that is non corrosive and can hold saltwater without erroding away will be fine. its just getting the water to the tank without having to haul it yourself lol. eitherway you rig yours up a pump in the bottom with some large diameter airline tubing will work out fine.