I ended up getting an Incase zippered cover, with a stylus pocket. Fits very well and the stylus doesn't really get in the way of the body of the BOOX, but it is close.
https://www.incase.com/devices/ipad/...-5-pencil-slot
BTW, the Remarkable has the following dimensions:
177 x 256 x 6.7mm
So a Remarkable slipcase would fit the BOOX 10.3 as well, but no closure...