Fixed barrel adapter washer

[Brett]

I have Rugged Obsidian I use on my 45-70 but the adapter butts up too tight to the barrel shoulder and I end up having to use pliers to break it loose even though I hand tightened it. I was thinking I need a spacer or washer, not a split washer, like was behind my A2 flashider on a 15-22. How do I determine thickness or where to get the washer/spacer?