West System epoxy seems to be fairly popular, and I used it myself to build a background. It may yellow slightly over time, but depending on what you're coating, it may or may not be noticeable. There are different hardeners, one of which is intended to be extra clear (207), but I have not used it myself.
Bubbling usually occurs as a result of the heat from the curing epoxy causing gasses to escape from a porous material, like wood or foam. You can try to minimize it by using several thin coats, to reduce the amount of heat produced with each coat, or by painting or priming your material before you apply the epoxy.