Banana Plugs too narrow

Are they locking bananas? If so, you need to tighten the collar after you slip them in. This expands the post for a tight fit.

any chance you can change out the connector for locking banana plugs?

It took several videos, but the interweb really does have all the answers. Bananas have split sides. Stick a screwdriver in there, twist it sideways, widen the slit. This will increase your girth. But you will lose a little length. Your banana will be thicker, but shorter. It just won’t go as deep, even when you are all the way in. Sorry. But in spite of everything you may have heard there is no way to make your banana thicker and longer. If you want to see how this is done do a video search for don’t break me. Or was it speakers love big bananas?

Tricky old audiophile? No, that's not it.

Don't worry. I'll keep looking!