There are sweat copper couplings available in most sizes at least down to 1/8”, the plumber in me would be more comfortable with a soldered joint than a compression coupling given the vibration, and the outside diameter of a solder coupling is much smaller.most plumbing supply houses stock this stuff.
For reference most small diameter soft (copper)tubing is sized by the outside diameter, 1/8”od, 3/16”od, 1/4”od, 5/16”od, 3/8”od are normal products
Chuck
