Apparently YouGov is the polling company that has provided the most consistently reliable Scottish polls in recent years.
Unfortunately YouGov doesn’t do Welsh Polls, mainly because there are not enough Welsh members to create a reliable poll. Wales is considered to be a part of the Midlands of England because of the lack of Welsh members!
There is no way that a YouGov poll can be hijacked by, say, 5000 Plaid members joining on mass, as the administrators carefully choose who is asked what question, but the more Welsh members, the more likely that Welsh issues can be addressed by the company.
Members of YouGov get paid for giving poll opinions (about 50p a throw). You can't take money out of the scheme until you have earned £50 (i.e. done about 100 questionnaires). How long it takes you to be invited to take part in a 100 questionnaires depends on your profile.
If you would like to join YouGov by clicking on this link, I get a backhander for enrolling you. If you don't want me to get that backhander, fine, Google will give you means of joining YouGov independently. But do join, one way or another. The more Welsh members, the better the chance of reliable Welsh polls.