What wood to use for linings? Willow is not readily available to me. I have some Western Red Cedar. Would tht work?
Use same species as your ribs. I'm sorry I've not been following your build. Are your ribs willow?
We are talking about less than 1% of the total material, so anything will work. Essentially you are looking for something that will increase the surface area of your glue joint. Spruce works great and is well within the tradition. I've seen almost everything you can think of for this purpose. I have a huge supply of 30 year old air dried 5" thick Honduras mahogany beams that I often use for blocks. It may not be traditional for basses, but all of my tests work fantastic and it is well accepted in the smaller instrument world. Kay proved that you can use tulip poplar for inside linings. Those basses may have a lot of issues, but I've never seen anybody say the problem is from the 60,000 basses they made with tulip poplar linings.
For inside linings, the main thing I look for is something that is stable and moves at a similar rate to your spruce top. If you are going to double up and use outside linings, something that either matches or is similar to you rib material. Plenty of my guitar building friends use cedars for inside linings. Structurally they are fine and it leaves a beautiful aroma when you open the case.
Surprisingly, you will notice quite a stability difference between quartersawn and flat sawn with the linings. Make up a few different test pieces and the right one will speak to you. Matching the rib and back material is a great idea for the back linings since they are usually the same wood.