The maybe not cheapest unless you already have decent tablet is... yea, fix tablet to middle panel.
It will require decent skill and hours of hand work with dremel but I think it's worth it. Later it's just drilling two 3mm holes in the back of tablet body for power source and audio cables.
Brilliant idea as already you have Wi-Fi, BT, 4G, GPS, big LED touch screen, ECU connection, any application you want from games and google maps to Torque Pro and other ECU readers.
Wanted to do that in my CDi, but had/have no tablet and good Pioneer radio with BT was much cheaper than tablet. Can't complaint.
It's not Smart but can give idea how it looks: