Resting the knuckles will lower the hands, and holding a neck strap will tend to create piano hands.... those go against what the op is requesting AND they straighten the elbows away from the trunk.
Sorry I must disagree with this. Resting knuckles/using a bucking strap is a tried and true method for getting the rider's muscle memory correct in positioning of the arm.
The upper arm should rest beside the upper body, there should be a slight bend in the elbow which changes as the rider posts, or the horse canters, etc, in order to keep the hands still relative to the horse's neck. The best way to train this is to have the rider use a bucking strap until s/he has good muscle memory. Some riders (myself included) who have stubby arms will find that they have to carry the elbows infront of the torso, this is not incorrect. What is correct is to have the hands quiet and independent. What is incorrect is to have the hands too high, working backwards and pulling (as is seen in 95% of north american riders), likely because of this "posed" thumbs up, heels down mentality.
I do not have a problem with "low" hands. However wide hands are a vice which needs to be stopped. The hands should be no wider than 4" apart at any point. By keeping the hands close together, they cannot drop below a certain point on the wither, they cannot be brought onto the saddle or the leg, and they generally stay in their "box". Riding with a bucking strap is the number one best way to get a rider to hold his/her hands close together and quietly.
Until the rider can effectively press the hands away from the body and use them in a completely independent fashion, the hand position will never truly be correct and the horse will always have a sore mouth from being pulled on. Learning by use of a bucking strap, pressing the knuckles, etc.. is the best way to learn to be kind to a horse's mouth.
Good luck OP!