I have seen nuts made from hardwood, and it would probably suffice for this rather-light weapon. However, some of the crossbow sites complain of splitting....
For this one, I'll use a design where the nut is actually held by steel flat-stock sidepeices, screwed onto each side of the stock. There are a lot of examples of similar designs, some using pieces of hardwood instead of steel.
My thought is eventually I'll be able to upgrade if I want to.
A couple of nylon (oops, not very traditional...
) washers on each side of the nut.
When I was stationed in Germany back in the 60s, the local (Wurzburg) museum had a wonderful collection of medieval weapons. Many of the crossbows were fantastically engraved and inlaid with silver and ivory; obviously not meant for a peasant!
Many of the bolts were very heavy, sort of bottle-shaped with thick center sections meant to add mass for armor-piercing.