Wouldn't it make sense to post the URLs, so that other users could also use your custom searches?
Double-click a custom search in Alfred Preferences, then "Copy URL for sharing".
You may have to use Code tags (the <> button) to post them, however, or the forum software will mangle the URLs.
For example, here's my TheTVDB.com custom search:
alfred://customsearch/Search%20TheTVDB.com%20for%20%27%7Bquery%7D%27/tv/utf8/noplus/http%3A%2F%2Fthetvdb.com%2F%3Fstring%3D%7Bquery%7D%26searchseriesid%3D%26tab%3Dlistseries%26function%3DSearch
Copy and paste the URL into your browser or Alfred to add it.
EDIT: Apparently, the code tags aren't necessary any more since the URL-encoding bug in Alfred was fixed.