Bar, Not Bench
When doing split squats, elevate your rear foot on a bar, not a bench. Why? Because it'll keep you from screwing up the setup, and screwing up the setup will limit your ability to train this exercise effectively.
Rear Foot Elevated Split Squat
Normally lifters will set up on a standard bench. But leg length and anthropometry are not standard, so it doesn't make sense for everyone to use the same height bench.
Too low of a bench wouldn't be ideal for a tall lifter (over the height of around 6'2"). But a problem arises when smaller lifters are using too high of a bench. As the bench gets higher, the back leg's range of motion becomes more limited due to mobility restrictions through the hip flexors. An overly elevated back foot combined with less than optimal hip flexor mobility places the spine into a hyperextended position, especially at the bottom of the range of motion.
Using a bar is a simple fix because you can customize the height of the back leg and get a full range of motion with a perfectly neutral spine. For the average lifter, this height will be somewhere between 12-16 inches.
Once you do this it'll be like taking the parking brake off your mobility. It'll also allow you to go heavier. Never overlook the setup. The better your setup, the bigger your results.