Garden Planter- what to fill it with?!

Hi folks. I’ve just assembled this huge planter (w = 2.6m; d = 0.4m; h = 1.0m) and now I’m wondering what to fill it with- any ideas? I’ve read a bit about Hügelkultur but don’t know where to find/ buy green/ undried logs. Essentially I’m looking for a cheap and easy to get hold of void former, rather than filling with 20+ bags of compost. Any ideas appreciated!