Central supports for bed
The existing supports look to just be a piece of wood stood on end, and fastened into the rail running the length of the bed. The simplest solution would be to build a wider support, also out of wood, and also fastened into that rail. Grab a 2x4, cut it into pieces ~6" long, and stack them until you get the height you need. 6 lengths of 2x4 stacked in this manner should get you the 23cm you need (a 2x4 laying down is only 1.5" thick, and 23cm is 9"). Screw or glue them all together and you're good to go. Orient them perpendicular to the rail for additional stability.

A single 8' 2x4 typically seen at the home centers will cost about $2 or $3 (in the US, at least), and a box of screws a few more dollars, making this a very cheap solution.