You dont say how old your learner is, but regardless I highly recommond a grab strap over the d rings as the best solution. Depends how old the rider is as to how big it needs to be though.
My neice has learnt to ride her pony with one. Ideally it needs to be long enough plus a little bit so both hands can be in the correct place when holding both sides of the strap but the strap isnt as tight as it can be so the rider has it under pressure doing nothing. We have her hold it and the reins as if she was riding with a double, so as she is only little we had to find a really narrow strap! If she needs to let go to give the pony more direction in anything but walk atm its only allowed to be one hand, and should be the inside hand opening outwards.
This method has double benefits as it means the outside rein is a little like a side rein and still, so the pony is much happier with it, and she learnt that forgetting to hold the strap and flapping the reins about results in her pony stopping, and that you dont have to use (much) rein to steer. If she looses balance she doesnt catch his mouth but having the strap just a little bit longer means she doesnt use it for balance unless she needs to and teaches her to hold her hands for herself.
It was a bit of trial and error to get it just right, but has been a game changer for her riding confidence!