OK, so here's a very rough draft of ideas for shields:
Shields have both a parry rating and an armor value. Using a shield is an Armed Martial Arts task. The parry rating adds to the character's Armed Martial Arts asset (not their die roll) when parrying. On a Formidable success, the attack is negated entirely. On a Difficult success, the shield's armor value is applied to the location hit (in addition to any worn armor). Parrying a missile weapon is one level more difficult.
The listings below have Toughness and Mass modifiers provided by shield type. These are applied to the Vehicle and Craft Construction Materials on page 38 of FF&S to calculate a shield's armor and mass, along with the addition of wood as a material on page 94 of World Tamer's Handbook. The calculated armor rating is parenthetical - if the equation generates a result of 5, the shield provides armor (5). Shields with double-digit armor ratings also provide armor 0 against firearms, and shields with triple-digit armor ratings provide armor equal to 1% of their parenthetical rating, i.e. a shield with a calculated rating of 12 is armor 0(12) and one with a calculated rating of 300 is armor 3(300).
Unaugmented humans can use shields massing up to 10 kilograms. Humans in Battle Dress (light or heavy) can use shields up to 20 kilograms. Schalli armor can allocate up to 16 kilograms to a shield.*
A shield has
Buckler: +0 to parry. Toughness x10, Mass x0.125
Examples:
Soft steel buckler - Armor 0(17), Mass 1
Bonded SD buckler: Armor 2(280), Mass 1.875
Viking Shield: +2 to parry. Toughness x15, Mass x6
Examples:
Wood shield: Armor (3), Mass 5
Fiberglass shield: Armor (4), Mass 6
Aspis (Hoplite shield): +4 to parry. Toughness x25, Mass x10
Examples:
Wood aspis: Armor (5), Mass 8
Fiberglass aspis: Armor (6), Mass 10
Scutum (Roman shield): +6 to parry. Toughness x10, Mass x8
Examples:
Wood shield: Armor (2), Mass 6.5
Fiberglass shield (riot shield): Armor (3), Mass 8
*This is of limited use under the current draft, since no shields are between 10 and 20 kilograms of mass, but I wanted to propose it now in case edits change the masses of available shields.