What form do I need to certify periods of insurance in other countries to claim unemployment benefits)?
You will need the so called U1 form.
The U1 form certifies your periods of insurance in another EU country, Iceland, Liechtenstein, Norway or Switzerland that will be taken into account for the calculation of unemployment benefits.
You can obtain it from the national employment service of the last country or countries where you worked and you should submit it to the national employment service of the country where you wish to apply for benefits. In the Netherlands you will need to contact the UWV.
The U1 form corresponds to the old E301 form.