I use 1/2" foam core or 4 cake boards stacked for moderate (3 tier) cakes.
For four tiered cakes on up, I order the 1/2" nova wood boards from Oasis Supply (cake supply company). They are like $10 for a 16" round board but if you are selling a 4 tiered cake then its not a huge expense and it will be the best base for that heavy of a cake. If the party or wedding is not far, you can get it returned to you but I never worry about it.
When I get those nova wood boards I always tape the sides to make the corners more gentle on the foil.
For wedding cakes and anything over 2 tiers, I use 1/2" mdf. We buy the boards and my hubby cuts them for me.
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