RE: This is gonna be tough...
If you go with swept wingtips [like a Taube] and angle them down somewhat, you should get away without using any vertical fin. I've got flying wings that have very small tip fins.
Are you using elevons, or elevator / ailerons?
On the tail, you could also trail and droop the outboard feathers to add some vertical area.
Landing speed will be slow, so the drooped parts are pretty safe with a strip of 2 oz cloth on either side of any angled glue joints.