Sorry for the delay, I was on a short holiday.
I think the issue is partially KitKat (as opposed to JellyBean) based. I can put in a request to the firmware developers, but there is no guarantee they'll sort it.
If the ShowBox author's just allowed a custom location for downloads, we'd be there.
In the mean time, I see two quick fixes as a possibility
a) We see if we have access to an JellyBean firmware for your X7 (let me know what the label on the bottom of your X7 shows), which should then allow the folder/path redirection apps to work, and so you'd be able to invisibly/transparently point ShowBox to your external storage
b) Copying over the downloaded files from internal storage to external. They may require an extension renaming before most media playing apps recognize them as a video, but MX Player might be comfortable to just have it thrown at it. If it works well enough, a script could probably be written that would semi-automate moving over the downloads, at which point ES File Explorer could be used to create a shortcut to the folder (assuming you're happy using a standard style Android launcher) on your homescreen.