Friends are quite happy with cement blocks. They also use alot of cardboard for mulch. I used to have access to lumber and frequently used 2x6's placed on side. You still have trouble where the edges meet, and so I guess it would be better to use in conjunction with the plastic mulch. I often thought about just digging a bit of trench and pouring concrete straight into it without forms, but a poured concrete curb would probably be ideal.

Sometimes it is a lot of fun being resourceful in the country. I was thinking about your edging, and I recalled I once used broken up chunks of an old barn foundation as an edging around my garden. It had the look of an old rubble ruin, which I guess it was, but I put a water garden there and a perennial bed, and it had sort of a timeless mysterious look to it. I would go around the edge with Round-Up when it needed it. Sometimes having a big country yard can be overwhelming with the upkeep.