When I'm doing a spacer I will put a slot in spacer with matching slots in the scale ends. These slots allow epoxy to form a "pin" extending from scale end thru spacer into other scale end to help hold it all together. Even when using a spacer like this maple between the Walnut scales I make sure the epoxy will flow thru the liner material from Walnut to Maple in a side slot. I have used brass pins extending from Walnut scale all the way thru the maple to other Walnut scale, but have decided that is overkill.