You can also go the super cheap route and get a generic one.
https://www.amazon.com/gp/product/B01MF5S1S4/
Yescom 30L Motorcycle Tour Tail Box
I realize the Yourban and Mp3 have different tail-box mounting systems, but the basic idea is the same. You dont have to go branded to get a top-box.
The mounting braket it came with didn't fit my Mp3 but it wasn't an issue. I made a cardboard template for the four bolt-holes where a mounting bracket would normally go. Then I used the template to drill straight through the cheap ABS-plastic of the top-box so it lines up with and matches properly. With the addition some some store-bought metric bolts n nuts and a bit of ingenuity with brackets its on solid.
Its been about 2 years and 9,000 miles later I have had zero issues. No plastic cracking, no leaks, good sturdy mounting with no wobbles. Its big enough to hold my mid-size full-face helmet (with intercom mounted) and gloves. Super handy.
Only downside is that I cannot easily remove it but thats never been an issue. Just make sure that when you put the bolts through the plastic you do not over tighten it. The plastic must be under some tension but not enough to really warp it or put pressure on it.