if you plan to use any liquid on the outside of the barrel, i would suggest mineral oil. it has a very similar refraction index to most glass materials and would almost completely remove the refraction of the outer wall of the barrel, but you would still have the inside surface to deal with.
making an acrylic/polycarbonate box around the length of the barrel, with a hole on both ends to seat the barrel would be real easy.
Weapons Technician / Gunsmith
Don't look at me, I don't know, lol ¯\(°_o)/¯.