Cornerpieces for TrekStor SurfTab Wintron 7

I've recently ordered a TrekStor SurfTab Wintron 7" tablet plus a
Hama OTG Tablet Bag with Integrated Keyboard (meant for 7").
However the straps could not hold the tablet, so I've constructed these cornerpieces to literally fill the gaps.

You can add two rubber bands inbetween the diagonal cornerpieces.
Now I just need to find a way to map the special keys (the keyboard is not meant for Windows tablets)