Hi there, based on what you stated, I presume you are looking for something   
   that stays in frozen state longer than ice. It would seem everybody wants to   
   give you smarty pants answers leaving the answer in a quasi-ambigous statement.   
   Let me be simple. Frozen US carrier of WW2. Took a long time to melt if I   
   recall correctly. This method is also used for permafrost anchoring in Alaska.   
      
   Interested?   
      
   Get a big pot perfer-ably a pressure cooker. Get lots of fine sawdust and lots   
   of rice boil the mash till rice is mushy and super sticky.    
   Give light draining. Water should be 1/3 of the mash.   
   Mash the mash into coarse gritty paste.   
   Once a wet formable paste, place into mold/vessel and place into a deep   
   freezer ( Normal freezer will do too). Once frozen, seal the new pucks if not   
   in vessel of some sort.   
      
   Frozen mash wood last the longest for the cheapest price. Heavier than gel   
   pack but if you have a semi decent lunch pail you should have frozen core for   
   about 1.5 - 3 days depending on insulation and outside temperatures and how   
   cold the targerted items    
   ie food / refreshments / simple neutrino detector materials are before   
   utilizing the frozen wood pulp pucks.   
      
   I hope this helps, albeit a few years late.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)