System 3 describes a process in their "Epoxy Book" (pdf download) where you add part of the total hardener to resin and allow this to partially cure. This leaves you with a high viscosity material to which you add the balance of the hardener when you want to uise it to get a full cure and presto, at least they say it works. Unaware of any other way to get a thicker clear curing product. You may want to call them to make sure it will do what you want.

Jim