I use a mounting block for every horse regardless of height. I bring the block to them before they are even tacked up and show it to them, let them sniff it, lick it, lip it whatever they want to familiarize themselves with it. At first there is no wrong answer, they just need to get used to the scarey plastic steps on both sides. I hold the lead rope and let them retreat to the end of the rope and then I get them to start approaching the block. If it's only 1 or 2 steps closer to the block, that's fine, they can stand there til they relax. Then another step or 2 toward the block. All the while I'm making noise by stomping on the block, picking it up and setting it down, slapping it, whatever. If it takes several sessions before they totally ignore the block as a been there done that think, that's fine. I also like to put a treat on the steps and let them walk up to the steps and take the treat. Seems to make them accept the block really quickly.
Once they'll stand next to the block with me coming up and down, stomping my feet, shoving it around on the ground next to them, pushing it under their belly to the other side and they'll totally not be bothered by it, then I bring it close to their side and lean over the saddle from it. I put weight on my arms and lean over until I can fully belly over and they don't get antsy or upset and don't walk off with me bellied over them. If they start to move, I just say whoa and slide down their side and pull them to a stop with either the lead rope or rein and take them back to the block. Again, just repetition, no wrong answers and I do everything from both sides. Then eventually I step my foot in the stirrup and put just a little weight in the stirrup and then step off. Back and forth, back and forth until it's no big deal and they don't move. If they move, I step off and bring them back to the block.
Since you say this horse has bolted off with you while mounting, I'd do everything in the round pen or in a corner or chute so he cannot bolt or move very far, just to begin with. Once they're calm about everything I still take them to a corner or wall to mount so they can't move off very far while they're learning. I also will pull their head around to me as I mount, so they only can move in a circle. The whole goal is to get them to accept me mounting them from a block, a tailgate, trailer running boards, rock, picnic table, whatever I can find, and not walk off til I ask them to.
My mare is well over 16 hh and I'm only 5'3" in my boots, and at 54 I'm not as flexible as I once was, so I HAVE to have them trained to stand next to something or down in a ditch for me to mount. I just keep working on it and even once I have them doing really well at letting me mount from strange things, I still look for things when we're out on trail and I practice at least once per ride so they never think it's ok to take off without me in the saddle and giving them the cue.
***I don't specifically use CA's methods or Parelli's or Lyon's but I use a little mix of all of them, whatever seems to work in that instant with that horse. Staying relaxed and communicating that things are no big deal with my body language is a major tool in the tool box.****