Sidewalls
Quotes are verbatim and attributed to their original authors.
Sidewalls cap the edges of the core, protect it, and transmit force down to the steel edge. Builders use one of three families — ABS, UHMW plastic, or wood — and the forum has drifted strongly toward wood.
Plastic: ABS and UHMW
UHMW is tough but heavy, expensive, and notoriously hard to bond; ABS bonds inconsistently. A common verdict:
Freshly cut plastic must be flame-treated so epoxy will grip: "these freshly cut sides need to be flame treated… just pass a flame torch over the material for a few seconds" (thread #147). Bonding UHMW to a substrate before attaching it to the core is one workaround for the bond problem.
Wood sidewalls
Maple, ash, and cherry all work, but dense tropical hardwoods have become a favourite for their bond and stiffness:
The bond is strong enough that "the wood fails before the epoxy does" (another builder, #3968). Two cautions: dense hardwoods are hard on router bits, and a stiff sidewall stiffens the whole ski — you may need to thin the core or drop glass weight to keep the flex you want.
Poured urethane
A fourth route, developed on the forum by chrismp, is to pour a urethane sidewall in place — cheap and gap-free, and covered in depth in the poured-urethane thread (#4279).
Finishing
Most builders bevel the top of the sidewall (around 7°) and leave a short vertical wall above the steel edge for durability; wood sidewalls can be left raw, oiled, or lightly stained for contrast.