I therefore prefer the plain rectangular pieces of cardboard with some suitably aluminized Mylar in it to the eyeglass shapes. Now that I have been to 29 eclipses, my feelings are shaped by the past experiences, and I saw several times, most recently in Aruba, the misunderstanding of what the eclipse will be like that I think was fostered by too much emphasis on the special glasses at the expense of the glory of totality. This would leave the decision and interpretation of how long to view the partial phases up to the person using the product, allowing room for error. The criteria is quite specific based on the transmission of IR, UV and visible light. They have to be safe for prolonged use.
Most people do look intermittently at the partial phases.
Each time there is a shape change or a new shape is introduced we must put it through testing. Certifications are issued for the finished product, lenses, paper, shapes etc. From this point of view we do not encourage or recommend the finished product be altered in any way. I don't think it is wrong, but I've never submitted it for publication. That said, Fred Espenak and Jay Anderson list several sources in the US and Canada, and one in the UK, for safe solar filters in NASA Ref.