I also had problems with urine on the entrance and leaking down through the front ledge and then down under the bag. I finally spotted one of my cats enter and then not turn around while peeing with her tail sticking out the entrance. Looks like others here have the same problem. Once this started happening it became more and more frequent. It certainly makes for a messy and smelly cleanup.
I have a solution that worked for me. I used box tape and created an internal "ledge." After cleaning the interior entrance way with wipes and drying it carefully, just stretch a length of tape (with the sticky side facing out) across the INSIDE ledge. It sticks there nicely so it will form a firm 1 - 1.5 inch lip barrier. Next, since I used clear tape, I stuck a piece of white paper on the sticky side so my cats could easily see that there was something new there. Finally, I stretched another length of tape across the sticky side and along the bottom ledge to keep hair or anything from attaching to the first piece of tape and also to anchor the tape a bit more and keep it well attached.
It's been 2 weeks now and the problem is gone! I thought my cats might tear it down (they could if they wanted to) but they just jump in over it. I also thought they might try to pee over (or all over) it, but nope. They also used to kick a fair bit of litter out every once in a while but that has stopped as well. The tape remains intact and unsprayed upon.
I also used to have the problem with one of them spraying into the waste ports (and therefore under the bag), but for some reason this has stopped as well. (I have my fingers crossed.)
Hope this helps others it certainly worked great for me! Good luck!