I'm with you. I also have very little trouble. Last year I had only one laggard, who really never specified a reason for late payment. And I asked several times if there were problems with the install I could fix. But he's paid in full now.
However, if you've never had a difficult customer, you're really fortunate. I had one a couple years ago that we're having to take to court to get payment. The problem that time was with me. They changed the design 6x. The changed the paving pattern 2x. They changed the materials 6x. They still weren't happy with the finished product. My mistake was, I should've seen it coming and ran for the hills.
But I like the Cert of Comp idea. We have a survey that goes out with the final invoice, gives them $5 off final bill if they fill it out. If there are problems on the survey we call them to ask what needs fixing.
Do you worry that you're inviting non-payment with the Cert of Comp? I'd worry that you'll find people that you'll never make happy.