You're never going to remove all of the risk, but the standard recommendation for keeping raw fish for a day is placed on crushed ice, and kept in a fridge.
As the ice will melt, you also need to make sure that the fish won't end up sitting in water. So, what you want to do:
- Find two identical plastic containers that when stacked together have a bit of a gap between them, and are tall enough to fit an inch or two of crushed ice, plus the fish, without it coming into contact with the lid.
- Punch holes in the bottom of one of the containers, and set it in the other one.
- Fill the perforated container with crushed ice, and then place the fish on top of it.
- Drain the water and replace the ice in the morning, then place in an insulated container for transport.
- Place the container in the fridge when you get to work.
For Poke, I'd place it into a sealable plastic container that would loosely fit into another sealable container, so that you can pack crushed ice between the two. Ideally, I'd look for an insulated tiffan-like container (wide mouth thermos, sometimes sold as a 'bento jar'), and then:
- Put into a suitable small sealable container, pack in crushed ice, and place in the fridge. Pack the outer transport container with crushed ice, and place in the fridge with the lid off ... or place in the freezer with the lid off. (we want to chill down the inner surface).
- In the morning, pack the outer container with fresh crushed ice, and set the poke container in it, and pack with additional crushed ice. Seal, and place into an insulated container for transport to work.
- Once at work, move the containers (so you still have crushed ice around the inner container) into the fridge.
You might want to look for poke recipes that are especially salty, as it will act as a preservative. If there's any acid in it, you may want to wait to add it just before eating, so you don't end up with ceviche.
