I was very disappointed in the way my buffalo wings travelled to a pot luck.
It was only four blocks away, we ate the appetizers probably 30 minutes after I fried them up, and I DIDN'T sauce them until they went to the table.
I had put them in an uncovered aluminum roasting pan and slid them into the hostess' oven rught when we got to the pot luck.
I, for one, will not try to transport fried food again. I'll have to choose something else.
Hope I didn't rain on your parade. There's probably some wonderfully simple solution that I've not explored.
I have a thermoelectric cooler that rides around in the back of my wagon. If I turn the plug connection over it becomes a warmer, and I suppose I could put an open vessel of deep-fried stuff in there and have some hope of preserving both heat and crispness, if I didn't have too far to go. Otherwise, the only way to preserve crispness is to let the food cool before you cover it. This works just fine with fried chicken, but in the case of tempura I think that, given the choice between hot and crisp, most of us would choose hot.
I don't think you can maintain the crispness of fried foods, especially if you're transporting it. You could drape an absorbent towel/paper towels over it to try to keep the moisture away but it won't stay as warm as if you cover it. If you cover it, it'll stay warm but you'll lose the crispness. You could top w/ paper towels and then cover it which I've found to be the best but still not great. Or, try a paper bag, like fast food french fries.