Karen Larsdatter’s Material Culture Linkspages are often a valuable resource for researching medieval material culture, and she has a lot of valuable material on this subject. Go there first.
Here is some additional material.
Flattened disk shapes:
A 13th c. canteen of brass inlaid with silver, of Syrian craftsmanship but decorated with Christian themes.
The flask of St. Rupert. This is a wooden field flask or canteen that reputedly belonged to St. Rupert in the 7th-8th century A.D. Around 1345-1400 it was given a case of copper and silver. Whether or not the wooden flask actually belonged to the saint, it must have been plausibly old when it received the metal case.
Here is the description from the Cathedral Museum of Salzburg
Feldflasche des Hl. Rupertus
Kunstwerk: Treibarbeit, Böttcherarbeit; Gefäße Haushalt; Feldflasche; Salzburg(?)
Dokumentation: 1345; 1400; Salzburg; Österreich; Salzburg; Dommuseum
Anmerkungen: 20x25; Katalog Dommuseum Salzburg, 1974, S. 54f
Objekt: Feldflasche; Holz
Teil: Beschlag, Hülse; Silber, Kupfer; getrieben
Teil des obigen: Inschrift (VASCVLVM HOC L (IGNE)VM RECLVSV IN ARGENTO FVIT SANCTI RVDPERTI)
Nativity, Mayer van den Bergh Museum, Antwerp, ca. 1400. “has stylistic affinities with Broederlam’s work, but cannot be positively ascribed to him” (Dupont and Gnudi, Gothic Painting) Melchior Broederlam was active 1381-1409. A silver colored field flask is shown in the right foreground, very similar to surviving 15th century examples in pewter or tin. The shape and relative thinness of the lugs is also more appropriate to metal than wood or ceramic.
A greyish colored barrel shaped costrel, possibly pewter or tin with raised decoration is shown in Broederlam’s Flight into Egypt, 1394-1399.
Pilgrim Flask Shapes for Domestic Vessels
Vessels with the flattened disk shape of pilgrim flasks appear in depictions of domestic scenes no later than the 14th century, Typically, the vessel is similar in shape to the flask actually used by pilgrims, with a foot and flattened disk body, but larger. This form would remain popular for centuries, in part because the flattened sides provided a congenial field for ornamentation.
A scene from Machaut’s Remede de Fortune fol. 55, ca. 1350, shows silver colored vessels of this kind in the right foreground. Like their prototypes they are equipped with straps that would have made it easier for servants to carry the capacious flagons.
The January page from the Tres Riches Heures de Duc de Berry shows a gilt version of this sort of vessel displayed on a sideboard.
A pair of silver-gilt flasks of this type from 1400-1440 survive at All Souls at Oxford: note the screw stoppers.
I know of no evidence of cork used as stoppers in England before 1530, although it was used for patten soles and fishing floats earlier. Surviving flask stoppers from the Mary Rose were made of wood wrapped with leather for a tighter seal.
The pewter costrel below was made by Robert MacPherson. The stopper was turned from wood and wrapped with leather.