How to keep fried fish warm and crispy during transport to venue?
Fried foods neither hold nor transport well.  They are best served directly from the frier.  If they do need to be held, a slow oven which will keep them dry (by allowing steam from the food to leave) is the best way.

There simply is no good way to hold, transport, and serve fried foods that will maintain the crispy quality.  Think of every delivery meal you have ever ordered of mozeralla sticks, chicken fingers, or even just plain french fries.   They simply are not the same as at the restaurant.

If you absolutely insist on serving fried fish, the best way to transport it would be in a single layer, in an insulated container, lined underneath with paper towels to absorb some moisture, and with vent holes so that at least some of the steam can escape instead of condensing back on to fish and making the breading even soggier.  

You could improvise such a container with plastic or foil takeout containers, punching (fairly large) holes in the top, and using blankets or towels underneath and around the sides as insulation.  Make sure you leave the ventilation holes uncovered, so that the steam can escape.

Still, you will have tension between keeping the food hot, which requires no holes and lots of insulation, and keeping it (somewhat) crispy, which requires allowing free air circulation, but would cool the food faster.

In the end, this is a situation where choosing another item, one that is friendier for holding and transporting, will serve you better.

Poaching the fish would be ideal, as would creamy or saucy dishes with the fish as an ingredient instead of the featured player.