Check this out under Conference planning and pedagogy: https://cassidyinview.wordpress.com/by-topic/conference-pedagogy-and-planning/feedback-forms/detailed-feedback-form/ for a template you can download and adapt. The link also includes some tips for adapting that form for your own use. Other forms are also at the Conference pedagogy/Feedback forms link.
Watch here for more conference and workshop/seminar related ideas. This feedback form will appear in an upcoming book about Conferences.