1. Although knotwork and interlace designs appear throughout medieval Europe during (and before, and slightly after) the Viking age, they have been heavily marketed in the last few decades as Celtic. It is difficult to find designs that the average non-medievalist will associate with Vikings instead of Celts.
2. Even designs that occur first or primarily in Scandinavia can still be found in Celtic art, thanks to Vikings going out and conquering places.
3. The ones that are definitely, totally, NOT found in any Celtic art I’ve come across just look Greek, FFS.
4. Runes and twisty animals are totally the way to go, here. Knotwork, while having the benefit of being regular and therefore easier to carve once you get the hang of it, just ain’t distinctive enough.