I think you want to balance the hams with the legs, if I'm not mistaken, keeping up with the much more powerful legs...not over build one or the other.
Best results seem to be a combo of stretching and actual direct leg curls. Can't do one without the other if wanting to reach full potential of leg strength. Working each leg , one at a time, seems also to get the best results.
Might suggest, as a ham warm-up, placing the toes on a 2X4/BB plate. Keeping the total back/spine straight, chest out and hands gripped behind the back. This is a bwt stretch only. Bend forward, knees lightly locked, as far as you can...and hold for a couple of seconds. than back up. Going to feel a different kind of stretch because of the toes on the 2X4. The calves will also feel it strongly. Try for 20 reps, or so. Actually, some athletes will do these throughout the day.
First exercise will be step-ups...on a bench, box or platform (adjustable types). Try for aheight of just below you knees. Some guy's do these with a BB on the shoulders, some holding a DB. I favor the DB's, but help yourself to which you like. Different influence on the hams/hips & legs from each version. Can't really say one version is better than the other one. Depends on body type. 8-10 reps
Second movements will be the one legged leg curl. Would suggest standing, as a first option, but again..your choice. 8-10 reps.
By far the best leg curl exercise would be from hands on resistance from a training partner, one leg at a time. A partner would hold one leg as you attempted to curl it up, allowing a slow contraction of the leg biceps. Can work this way with both positive and negative applied force to the hams and in the same set.
Generally I would add Romanian Deadlifts for hams...but you have a back issue. RDL's are a great movement for hams, hip and lower back. Just to note, strong hams can add to your squatting power, which most guy's find out to their surprise.
Good Luck