One material I haven't seen recommended is stabilized rammed earth -
not for the OP, but they could presumably hire out the labor. You'd
build a simple, low open box (just 4 walls). The advantage you get
here is the mass will help warm the bed earlier in spring, stabilize
temps in the fall.
Stabilized earth is a clay mix with 5-10 percent cement added to it,
compressed using simple form works (in Africa, two boards are held
together with rope). Pound a "lift" of material 6-8 inches until it
rings, repeat until desired height is reached.
Cement is bad, but you are not using a lot of it, and it makes it
possible to create the raised beds out of normal construction site
waste (subsoil). Nothing else needed, will stand up to rain/snow. If
you live in a freezing clime, you will need a base of gravel to
prevent frost heave.
This is a LOT more work than the other suggestions, but I was struck
watching a DVD of monks who tended a garden. Their garden had some
low, 1-wall beds that warmed early in the season, and they were able
to shovel snow off the beds for planting early crops. Really
Offered in the name of the radish spirit!