I was perusing my favourite web store and they've just added this https://www.varusteleka.com/en/product/finnish-external-frame-rucksack-green-surplus/63316 . The pack itself looks a bit ugly but I'm thinking the frame and leather harness are ripe for modding. I'm thinking an Italian mountain rucksack with space below might work. Anyhow, I've ordered a grade 1 one.