So we are reenacting a siege around 1471 under Charles the bold, Duc of Burgundy. The best sources that we have for this type of construction would be a citation in the ordinance of Louis XI of France, paintings by Memling and illusrations in René of Anjou's "Livre des tournois". As stated in the ordinances, a jack would be padded with multiple layers of linen canevas or skins ;
"And first they must have for the said jacks, thirty or at least twenty-five, folds of cloth, and a stag's skin being the best cloth that has been worn and rendered flexible, is best for this purpose, and these jacks should be made in four quarters. The sleeves should be as strond as the body, with the exception of the leather, and the arm-hole (assiette) of the sleeve must be large, which arm-hole should be place near the shoulder, that it may be broad under the arm-pit and full under the arm, sufficiently ample and large on the sides below. The collar should be like the rest of the jack, but not made too high behine, to allow room for the salade. This jack should be lace in front, and under the opening must be a hanging piece (porte piece)of the same strength as the jack itself. Thus the jack will be secure and easy, provided there be a pourpoint without sleeves or collar of two folds of cloth, that shall be only four fingers broad on the shoulder; to which pourpoint shall be attached the hose. Thus shall the wearer float, as it were, within his jack, and be at his ease; for never have been seen half-a-dozen men killed by stabs or arrow wounds in such jacks, particularly if they be troops accustomed to fighting."
Not wanting to invest *too much* and still wanting the protection to be effective, I decided to go with tefflon padding and leather left-overs as interlining. Tefflon is sturdy *and* fire-proof. I've made a few tests by punching into the many layers and by slashing it with knifes and swords and it appears to be working pretty well. So I went for it. Even a big knife slash only manages to cut through the first of many layers. The arrows that we were using, even at a close range, would simply bounce back. In other words, Perfect ! :)