Based on the community measurement, a Lee 100mm system adapter ring is about 99mm outer diameter, give or take less than 0.5mm. That suggests it should fall within your matte box’s 95–105mm clamping range.

Only a Lee ring was specifically measured in the answers, so treat Cokin Z as unconfirmed unless you can verify it separately. If your matte box clamp has very tight tolerances, it would be safest to measure the exact ring size you plan to buy before relying on fit.