A good fence to use for working on straightness is a Swedish oxer (or St Andrews Cross), like this.
It tends to back the horse off a bit more than a plain oxer, and if you have a horse who tends to drift left, then you’d build it like this, so that drifting left put it towards the higher part. This way the fence does some of the work of convincing the horse to stay straight.
This fence is not allowed to be built in warm-ups.
