Kudos !
This is by far the easiest DIY background I've seen on the internet, great job.
The absolute winning point here is that it is actually removable,
on the contrary of the vast majority, if not of all, of the other displayed DIY backgrounds online.
One idea so that it doesn't move one way or the other (for example when fullfilled with water),
would be to simply add a point (about the size of a tiny nut) of silicon glue at each side
(right and left) of the tank, just against that DIY background.
Thus, it will stick to the back of the viv/aquarium,
and if you ever decide to take it off of your vivarium/aquarium,
you just cut the silicon "points"/"nuts" out.