Just about any of the small ones that you put on your tongue. A raspy turkey call will work too. Just like calling turkeys, it takes a little practice.
Get a cow call and use it about ten times as much as you bugle. Never bugle while you're moving in on a bugling bull. If he has a harem he's likely to keep moving away from you. And, if he has a harem he's not going to be coming to your cow call. In the natural order, the cow goes to the bugling bull so you can move in pretty close while pretending to be a cow. But, unless you look like a cow it's a challenge to make the final close.
Others will disagree with me, and that's just fine. I'm not an expert but just going on my observations over time. There's probably better ways of doing it so keep an open mind.
The other thing is, you can't tell the size or maturity of a bull by their bugle. I have brought in several lone, mature 6X6 bulls that were squeaking like juveniles.