containment boards never work. they let the problem fester and then spread out to the site as a whole (if they live) or nobody uses them (if they die)
most users don't use other boards as it is. an /idpol/ board would not be a board for /leftypol/ to discuss idpol, it would be /idpol/, an insular community that most /leftypol/ users don't post on. if i personally wanted to know /leftypol/'s views on an idpol topic in that situation, i'd risk a ban to make an idpol post on /leftypol/ rather than getting the pointless opinions of /idpol/ on the matter.