Since a while it's now impossible to chmod +x on a usb key, which is required for a real portable app.
See here :
https://ubuntuforums.org/showthread.php?t=1686624

So running fopnu extracted from zip version which is the only one common for win and linux (You know, one Key to rule them all), and starting from a usb key seemed impossible. After searching, I finally found a work around, a little messy, but it runs.

1 rename the app on the key fopnu_LinuxXbit by adding .com (I know, we are on linux, but so it is required to allow the X flag : only possible for .exe, .com .bat and script text file starting with #!)

2 remove the key.

3 create the file sudo touch /etc/udev/rules.d/90-usb-disks.rules

4 edit it and past in these lines :
# UDEV Rules to change the permission of USB disks
#

KERNEL=="sd*[0-9]", ATTR{removable}=="1", ENV{ID_BUS}=="usb", MODE="0000"

5 restart : sudo /etc/init.d/udev restart

Then plug in the key, find the app, right click on it and modify permissions to execute.
Now the flag is set !

Sorry for the mess, but as it works (tested on Xubuntu Xenial), I didn't dig further.

Hope to spread fopnu !

DB