An Advisory Board brings clarity in decision making; and best of the CI (competitive intelligence) backed market aspects, especially for a firm entering a success/growth cycle between 4-8 years.
It takes 3 pillars to build a strong roof of an Advisory Board:
1) Your offering (Services or Product): Rule of the thumb says, shortlist members who have expertise in the field of your offering; or to the least think of fetching someone which matches the horizontal integration of industry field.
2) Exposure and Numbers: My magic number (of recommendation) for you is 1.5 members per operational year of your business, once you cross 2 years. There are many factors to it, however given your growth and operational years (presuming all 5 years), you shall look for 4-5 advisers for your ''Advisory Board'.
'Exposure' is synonym to your characteristic point: Avoid approaching people whose skills/expertise is only limited to Law and Finance (Accounts and Investment Bankers) - but I believe you know it already as a universal rule. Always be open to people who are not hungry for money and part share of your company, but really interested and see your vision/mission a real hit. This leads to my third pillar...
3) Clear, Brief & Straight Agenda: Never, never, never approach any potential member with an open-ended agenda. I personally have this filter while reading/meeting people around. Tell them (in brief, watch no. of words below) what you are planning, how much you have achieved, where your target/milestones is, and what hurdles you need they can help you overcome:
Express your very first approach by using a simple pitch (in person or in written or in a presentation):
100 words: What your company do? Customers? Market Size?
50 words: Few quick statistics to help them envision your achievements and/ or under-achievements.
150 words: Purpose of approach. This is where you shall describe your problem/ targets/ mission/ milestone to achieve; including where so you think that member/adviser could be of help (link this to his background/exposure i.e. why you shortlisted him/her for this potential alliance)
I hope it helps. Good Luck.
Feel free to reach out in case you need a thorough discussion.