Hi Dmitry,
You could do this in Chimera using volume style display, making it 100% transparent, and using orthographic projection (instead of the default perspective projection), e.g. using Chimera commands.
vol #0 transp 1.0 bright 1
set projection orthographic
You’d also have to choose the brightness for each map value by adjusting the yellow brightness curve on the histogram in the Volume Viewer dialog.
If you really want to get something for quantitative comparison to class averages you should use other software intended for that purpose.
Tom