I've been busy as you like over winter and spring creating raised beds all over the plot. What I've used is 2nd-hand, unwanted, scrap....... concrete panels, the type used between concrete posts for typical fencing, sometimes used just as a base panel or two and topped with waney-lap wooden fence panels. You must know the type I mean. Standard 6 feet long, 12 inches deep.
Admittedly it is much harder work than using planks or boards, but once done.......
I have a mate who has a fencing company and who is prepared to put aside concrete panels (and posts) that would otherwise go into a skip. Most of them are chipped, maybe even a slight crack in them or are discoloured with age. They have either been renewed by customers of his, or torn down entirely and replaced with something different. There's always loads of them about if you look hard enough. Enquire at fencing companies. They're happy to get rid and save on mountainous skip costs.
So obviously you need a van to transport them, enough muscles to lift them and carry them onto site and serious hole digging equipment. I've been cutting the posts (petrol Stihl saw) into 4 foot lengths. I got enough posts early as many 6 and 8 foot posts are reclaimable if you only need 4 foot lengths - you just cut off bad ends and can always usually get a good 4 foot out of the longer lengths.
I haven't concreted the posts in - hold on a minute, there's enough work with them as it is - but have dug them all in 2 feet deep, leaving 2 feet protruding above ground. You can then slide in 2 panels, giving a genorous 2 foot raised depth. Probably this is overkill and 1 foot raised would be enough. You can always just slide in 1 panel to start and put a 2nd panel in as and when you come by them. Once the panels were in place I packed some excavated pure clay against the outers to help retain them, not that any have really moved much anyway.
Hard work, but once done, it's done for ever - they won't rot like wooden panels eventually will.