I used scaffold boards, kept the 13foot length but cut them into thirds for the sides. I don't mind walking up and down them but I only have 4. I treated them with green wood preservative before use. I put little stone paths in between, which I admit are too narrow really but that's the only thing I would change. They are in my garden ( no allotment) and they look really nice from house and up close.
I filled them with contents of my old compost heap and any soil dug up from paths. They do take a lot of filling, 2 years on they are not full up but i prefer not to introduce anything into garden unless really necessary. Not much help immediately but you can throw in your used tomato compost in autumn ( best put in bed you had your spuds in this year as toms are same group for rotation )