Andy,
When new, the Unicolor gaskets are nice and pliant. Over time and with usage they become stiff and lose their resiliency, resulting in leaks. In the color section of the lab I worked in we had quite a number of Unicolor tubes under heavy use, and frequently replaced gaskets. I still use quite a few Uncolor tubes for my own work and have had to replace every single gasket.
The best material to use is thin neoprene (about 2mm thick), which you can buy in small sheets in a crafts store. They come in every imaginable color for kid's art and craft projects, including the mandatory black. A dollar for a sheet about 12x12 inches, enough material for a number of tubes. Use the old plastic gasket as a template, and cut a simple ring. You can dispense with the radiating tabs that come with the original gasket. I use an inexpensive circular cutter for the job, although a pair of scissors would do. 2mm is actually too thick to permit easy installation of the Uniroller lid, so what I do is I use a hammer to flatten the neoprene ring a bit. This also breaks in the material for a better fit. Remove the dried up original gasket, then rubber cement the neoprene ring onto the rubber ring seated in the perimeter of the lid. Install the lid. It will be tight at first. The let sit for a few days for the neoprene to break in. In all the years of refurbishing Unicolor tubes this way, I have yet to have a single leak. I too have tried using coffee can lids as replacement materials. They leak!
