3/8 MPT washer size

[jumbo_tron]

Hey guys I'm working on building a weldless sightglass for a kettle.
I will be using 3/8" compression to 3/8" MPT 90's with polycarbonate tubing between them. I will be using the 3/8" NPS locknuts from bargainfittings but need some washers, I'm not exactly sure what size I should be looking for I measured the largest diameter of the threads and it was about .625 to .675 inches. I have found some at grainger and mcmaster-carr but I would like to know if anyone else has found some that fit for sure.
Thanks

 

[jumbo_tron]

Well I've been looking some more and I think I'm going to try some 11/16" ID washers but the only problem is the smallest OD I can find them in is 1 3/4" which is actually a larger OD than the nice thick 1/2" washers most weldless kits come with. I'll keep this updated with my results in case someone else runs into this issue.