I took a similar approach. I just was working on this last week. I put the mounting block so I had a "lane" between it and the fence. That eliminated one direction she could move away. Then I had her in her rope halter and led her to the block, told her to stand, and stood up on the block.
I did all sorts of stuff besides mounting. Scratching her butt, patting the saddle, putting my foot in and out of the stirrups, messing with her mane, etc. If she moved, she went forward, I could pull her around and direct her back and forth through that "lane" until she stopped nicely at it. This way she couldn't escape the block by being naughty.
When she did stop nicely at it, I gave her what she wanted. I got down and scratched her ears. Then I got back up again and we worked on it again, until I could get on her, then immediately get off her without her moving.
Once on her, I spend a minute practicing flexing so she assumes the first thing we're going to do when I'm in the saddle is stand and flex, not move off. I don't want her to anticipate moving off.
Once she was solid near the fence, we worked on it in the center of the ring. Now she is doing really well.
Word of caution, if you have a horse that pulls you with a rope halter on, you want to do more ground work, because you don't want it to pull you off the block and hurt something.
Anyway, it's working for me.
Make the right thing easy and comfortable, the wrong thing uncomfortable, and remember to reward the first few good behavior with what the horse wants, instead of what it doesn't want, (you hopping on its back and working)
Good luck, and patience!