I use one of these. Just need to makes sure it fits underneath your top jacket and the top jackets fits.
Dry Evaporative Cooling Vest - Large S-24068-L - Uline
Stay cool and dry. No soaking or inserts required. Provides up to 72 hours of relief. Fill with 13-20 oz. of cold water. Mesh side panels for ventilation. Zipper front.ULINE offers over 41,000 boxes, plastic poly bags, mailing tubes, warehouse supplies and bubble wrap for your storage...
www.uline.ca