A traveling scholar during the Dark Ages, with full knowledge of how precious books were, decided to keep his rolled up in a scroll. He's fashioned a clever case for it that allows him to turn a nob on the exterior, and then the scroll unrolls partially inside. The case also contains a window, such that he can unroll the scroll inside the case, and then read the text through the window.
The case is considerably more complex, and is formed from a series of rollers, allowing the scholar to roll through the parchment until he finds his desired text, and keeps the remainder of the parchment rolled up tightly. The case itself is sealed from the outside, and is perfectly waterproof. But the specifics of the case, beside the window, aren't involved here.
The window is the problem. Clear waterproof plastic is a common material nowadays, but this is set in the Dark Ages, otherwise known as the Early Middle Ages (400-900 C.E., approximately). Most convenient modern materials didn't exist back then. Glass did exist back then, but it was prohibitively expensive, and the scholar would really like it if his manuscript only costed one fortune, and not two. (Also, glass is fragile.)
Is there anyway to form a clear waterproof plastic-like material using Dark Age technology?