Enrico Granata has graciously provided a free tool to create container files for virtual VAX and Alpha solutions. Called MIM (Make IMage), it is run from the command line and quickly creates an empty container files of a user specified size. MIM is intuitive to use. Simply run MIM at the command line to obtain usage instructions.
MIM can be downloaded from the FreeAXP page. Look for it under FreeAXP Links.