How can I display the border of sub-regions of a map with box?

Dear All,

I created a map about Europe with observed longitude, latitude and elevation data and small maps about some region of Europe with the following functions:

% Europe
axesm('lambert','maplatlimit',[ 26.5861 71.8699],'maplonlimit',[-34.748 60.8422]);
pcolorm(lat,lon,z);
demcmap(z);
h=colorbar;
set(h, 'FontSize',12, 'FontWeight', 'bold');

% Mediterranean (MD)
axesm('lambert','maplatlimit',[ 36 44],'maplonlimit',[3 25]);
pcolorm(lat_md,lon_md,z_md);
demcmap(z_md);
colorbar;

% Iberia Peninsula (IP)
axesm('lambert','maplatlimit',[36 44],'maplonlimit',[-10 3]);
pcolorm(lat_ip,lon_ip,z_ip);
demcmap(z_ip);
colorbar;

% Mid-Europe (ME)
axesm('lambert','maplatlimit',[48 55],'maplonlimit',[2 16]);
pcolorm(lat_me,lon_me,z_me);
demcmap(z_me);
colorbar;

% Eastern-Europe (EA)
axesm('lambert','maplatlimit',[44 55],'maplonlimit',[16 30]);
pcolorm(lat_ea,lon_ea,z_ea);
demcmap(z_ea);
colorbar;

I enclosed the results. I would like to create only one map about Europe and display the border of these sub-regins on the map with boxes. How can I carry out this issue? Thank you for your help in advance!