I used Minwax Helmsman Spar Urethane and mineral spirits (because spar urethane is oil based). I picked up both at my local store for about $15 and had lots left over. I used hardly any at all (although you could do more than one coat).
This was easier to apply than clear coat, just used a lint free cloth and dipped it in the mixture and wiped it on. Easy as pie.
The reason for the mineral spirits is to make it thinner and easier to go on, you don't have to worry about streaks because they smooth themselves out if you don't apply it too thick. If it gets streaked or there are bubbles or dust just use the mineral spirits to wipe it off and clean the lens. Wait for it to dry and do it again.