STiG, what a massive undertaking! I'm sure it will be wonderful when you have finished.
I'm still at the "concept" stage for our raised beds [I'm holding out until we can afford to get the ground levelled, because I think that will make life so much easier for the rest of our gardening lives], so I'm loving hearing about your progress. My husband thinks "scaffolding boards", but again, I've got my eye on chestnut railway sleepers, so that's another delaying factor.
May I make a massive hint for pictures? Or even a separate thread? [I miss Mel's Adventures in Aquaponics, so if there was a STiG's Crazy Half Acre thread, that would be a joy.]
You probably already have the soil/compost situation sorted out, but I came across this video on YouTube, which shows tips for saving money, and offers a sort-of hugelkultur option for filling a bed. We know from neglected piles of chippings/brash [too much to do, never enough time], that all manner of "rubbish" will make lovely compost.
I don't know how I came across the video [I think it was googling], but if you already posted that upthread/somewhere else, and I've just recommended back to you
your recommendation, I'm really sorry.
[Sometimes lasting side-effects (memory loss/inability to reliably form new long-term memories) are the price one pays for life-saving medication.
After six years off the meds, I don't think it's going to improve any, but it can make me feel like an absolute dimwit, so I'm
hoping it wasn't you.]