Stop smart cover from turning off the screen when folded on the back of the device

The "smart sleep" function is a convenient battery-saving feature that turns off the display when the cover is closed, but with the cheap cases, that the magnet is too strong and also turns off the screen when the cover is folded behind the device.
This can be easily solved at very low cost.

The smart sleep function works with a combination of sensor (in the device) and small magnet (in the flip case).

When the magnet in the flip case is too strong, it is also detected by the device when the cover is folded on the back, which causes unwanted screen "black-outs".

While there is no way to actually block a magnetic field, it's possible to reroute the flux by shielding the back of the tablet with a material that has a good magnetic permeability.

Question: where to find a thin, small and light plate with these magnetic properties?

Answer: The magnetic phone holders on eBay!
These kits cost around 1€ and often come with 2 of metallic sticky plates which are exactly what we need. When possible, prefer the ones that include one square plate.

The fix:
Locate the device sensor by moving this plate between the screen and the flip cover until the screen does no longer turn off. Then just report that location on the inside back of the cover and stick it there. 

Note: These plates can be purchased separately but are usually more expensive than the full kit.