Hi Jeff,
I like to use o-rings also, rather than tubing (unless the OD is too large for the nut ID, of course.)
Have you considered a metric o-ring? A 5mm ID x 7mm OD x 1mm wide might work. You would need around 9/32" dia. clearance inside the compression nut. I have a bunch of them, and could send you a few, if you think it's worth a try.
Paula