Good info all through, but pulling this line out...
Yes, I think that's where I was going, somewhat clumsily. I watched the YouTube video you referenced and Crawford's explanation does sound very much like a casting of lots rather than runes providing some inherent powers of divination.
And I think there's a view of modern rune casting, as @silveroak characterises, as throwing fancy scrabble tiles down, and I think that's the image I was struggling with as reflective of in-period divination.
The concept actually of creating lots, chips, or blótspánn with words or phrases (in the runes, of course) feels more atmospheric to me - and that's what I'm really after.
To model it, it's a form of Cleromancy, but the flavour comes in via the addition of runic words or phrases to the lots that are cast. I think I'm going to go with that.
Thanks all.