the flatter the better.
As a weight I use hard cover book that is bigger than the box, but not that big that it woulkd rest on the edges of the scanner.
Then I put some more heavy books totalling maybe 4-5 kilos.
I re-checked all scans:
The front ones seem to be that much out of focus that they are pretty useless., back ones could probably be combined to make one usable image, label are scans are very usable.
But do scan front and back again with heavy weight, and with few different settings, so we can see what is the best setting to use with your printer. And you can leave extra space around the image, maybe half inch.