Similarity evaluation

Hi,

I want to perform a study in which participants are asked to rate the similarity between two packages of different brands. I have been searching the literature but I have not been able to find any scales for measuring the similarity between two objects.

The only thing I have found so far is that participants have been provided a sample of products (e.g. 5 packages) and been asked to mark those that are most similar. As I will study the similarity between multiple product categories, this method is not practically feasible.

I am therefore on the search scales that have been used in prior studies, anyone that has a suggestion?