i don't know if this will help, but our Eliza (21) was missing the box and I moved them away from the wall(s) and the one that is under a shelf at the end of our upstairs hall didn't have enough room heightwise. Or so said the wife, so I raised the shelves up 6" for more headroom. That did seem to help. Eliza seemed to do better after those rearrangements.
I know exactly what you are talking about, because my two boy cats do this to and it drives me crazy too. I'm forever having to scrape the sides. There are two separate sides where they do this. As much as it drives me crazy, I have learned to live with it. After all, they are peeing inside the litter box.....hey, I'll take that! Having had issues over the years with this one peeing here, and this one not liking this kind of litter, this is doable. I still gripe, as I'm scraping down the side every time though
I have this problem as well. I finally bought 4 new very high sided boxes from Petco with lower fronts and put puppy pads under the fronts. Works very well. I used to get pee run all down the sides and under the boxes and it just about ruined my upstairs floor finish. No more problems.