There are a number of rather fine fics exploring the subject, including one by SectoBoss in which Onni has set up heavy defences on Tuuri's haven, and several tales (don't offhand remember the author, might be Aunty Agonee?) in which Lalli, and later Reynir, find Emil's haven, which is a Saxon-style barrow. Well worth a read.
*Edit: my take on the accessibility of havens is that physical distance and emotional closeness might both be factors.