Two modifications I did to the print bed:
1. I put heat conduct paste in between heating plate and aluminum plate.
2. When I tried to level the print bed, I realized that the aluminum plate is very uneven. So I first tried to rectify the plate with limited success. I got it even within 0.1mm by bending, but still the results were not satisfying for me, so I decided to add a rather thin glass plate on top (4mm). I cut a glass plate from an old window and wiped the edges with sanding paper.
As usual I use a glue stick to prepare the glass for printing. However I do not remove the glue afterwards, but just put a few droplets of water on top and with a small sponge I re-solve the dryed layer and make it even again. This way I don't need to re-apply the glue every time.