Wolf - I used my neighbor's mixer to mix concrete for a sidewalk.
they are available in 1.5 and 3.5 cubic feet size for the DIY guy.
check them out at Harbor Freight's website.
another option would be to lay out a big plastic tarp. spread the grains
out and mix with a garden rake. that would not damage the grain heads.
or - if you have a large area in your garage or carport, make a clean spot
on the floor as a mixing area.
and as stated above: get one of those big blue or white plastic drums with
the removable lid. (check craigslist). put in the ingredients and roll around
in the yard to mix.
(if you have young kids, get them involved in the mixing process).