Here's a "drop-in replacement for the standard <select> with multiple attribute activated": http://loudev.com (github). I've tested this script: it is well coded, has useful configurable events and has a decent size once minified (about 10ko). It is also working fine on mobile devices. After a few css and js tweaks, the integration in XenForo looks very good. It is recommended if you have enough free space on your web page (it's using two windows). Otherwise, if you need more compact layout, you can have a look at this other script: jautochecklist (already integrated in this plugin).