Lonewolf, over the past week or so I've been experimenting with a short nap and a longer core sleep. The result is that a slight reduction in the nap length equates to a large amount of oversleeping.
As an example, previously I had a 1.5/4.5 hour split with no problems. Reducing this nap to 10-15 minutes meant oversleeping 2 hours last night, and a similar amount the night before.
Perhaps by increasing the length of your nap to 90 minutes or so you will greatly reduce the length of core sleep you need. Try a 1.5/7.5 hour split for starters, and gradually decrease the length of core sleep until it feels right to you (and obviously doesn't affect your recovery).