This simple two penny coupler could come in handy so I'm filing it while I have it handy. We had a discussion about making and using gimicks. Twist two pieces of insulated wire together and use it as a capacitor. It works but to adjust it you have to trim. twist , or untwist it. This one uses binding post but you could thread the wire (12 gauge is the size of #4 screw) and make a screw adjustment.
Here we have another way to core a transformer. I've used this one and the toroid. They both work. I don't know if one is better. Maybe some time I'll build two cores and wind them the same then use toroid on one and this method on the other then have something to compare.
EDIT: This method is easier to provide an air gap. The toroid is easy to install and works well in circuits that don't require an air gap.