I am planning on making a sun shade for my tablet and I have some thin lightweight aluminium sheet spare. Was wondering if it would have any effect on the signals from/too antenna? It will have a wide slot cut out of it so it won’t actually be covering them.
It May even improve it 
Yes.
It can reflect, deflect, alter the beam pattern and all kinds of unpredictable issues.
Even leaving a cutout won’t really help as you’re creating reflections and overlapping interference patterns in the uncovered area.
Basically, don’t use Alu.