I'm a fan of connectors. (We own the "perfect conextion") However, I don't like how it inhibits a full swing.
I bought a "reverse connector" (my terminology)
Its an elastic band connected to two straps around the batters bicep area. It forces you to stay spread and compact to contact without the mechanical bar holding you back while getting through the ball. It works by applying muscle memory into the equation. The elastic is trying to pull your arms together but you have to keep them spread with muscle. After a few dozen swings, its like magic. The arms automatically spread without even trying.
I found it on a site called baseball rebellion. The product is called the bat drag buster. I paid $30 for it but had I known how simple it was, I could have been made for under $10
It is IMO one of the best tools ever made for teaching a correct swing. We even use it in live BP with no issues at all.
Investigating the idea of a "reverse connector", I came across a way-too-stretched-out elastic hair band in my house (free!) that turns out to be a perfect length to serve this purpose. Having to exert a small amount of outward muscular force to maintain the triangle makes the feeling really jump out. After trying it myself for a while, I had my 10yo DD give it a try, and immediately, she could "feel" the triangle better than she has before.
Thinking more about this, it makes a lot of sense. We're trying to get the hitters to maintain separation between their elbows. A rigid "spreader-bar" connector device requires/encourages inward muscle pressure, which is what we're trying to get away from. Teaching them the feeling of maintaining the triangle is more effective with an elastic band device because it encourages/requires different muscle use (IMO, YMMV)