U32 len = sizeof( U32 ) * some over estimate;
U32 *diffs =  malloc( len );
U32 i = 0;
SV *packed;

SvPV_Set( packed, diffs ); SvLEN_set( len )

for( ) {
    for( ) {
        diffs[ i++ ] = x;
        diffs[ i++ ] = y;
    {
}

SvCur_set( packed, ( i-1 ) * sizeof( U32 ) );

return packed; // sv_2mortal?