Sacha writes
I'm interested to try one or two in the tree stumps, as I said but they
might not like that.

You could try the ones that particularly like trees, polypody for
example - fairly simple fronded, fronds about 8 inches, not like the
maidenhair fern that you have (it's a tough looking thing) but different
from the usual male ferns and so on.
--
Kay