I've put a PEI sheet on my bed. Works very well.

I have also tried replacing the original plate with a glass bed. While it worked fine, you lose the advantage of quickly taking out the print surface.

In the end, none of these solve the leveling problems that come naturally with the single point calibration and the lack of corner adjustment. But most of the time I found the bed to work surprisingly well, for what it is. Yes, large prints (and I mean large as in what's possible with the somewhat small size of the machine) may cause problems, but on the other hand, the Voxel has become my go to machine for smaller objects such as miniatures.