I don't think I'd know how to! :-)
There are the odd few sites in Wales and England on Canmore. I wonder if they plan to put more sites from Coflein on Canmore? Be good to have them all on one map search.
Example http://canmore.org.uk/site/294988/hirnant
Both England and Wales now have their own (entirely different) "portal" maps that return search results from several different databases:
England:
http://www.heritagegateway.org.uk/gateway/advanced_search.aspx
Wales:
http://historicwales.gov.uk/#zoom=0&lat=300000&lon=258000&layers=BFFFFFTFTTT