I have done it. I assemble and freeze, bake when needed.

If there's meat in it, bake then freeze. With potential travel misadventures, you don't want otherwise forgettable pathogens to assert themselves. Highly recommend several Polar-Tech Re-Freez-R-Brix, which are foam, and superior. 3 for $15 at Amazon (informational—they may be easy to get locally). Supplement with dry ice, rather than cubes, if you can. Make sure to fill your cooler. If you have extra space you can freeze disposable containers full of water. Don't include travel snacks/drinks in the same cooler so you won't be opening it often.

If you can make it there without problems, that's best, especially if you can buy the fresh ingrdients there. For literally a couple of days—less than three—don't freeze the ingredients, including the meat. Make the container ice supplements, wrap the ingredients well so they don't get damaged, and pre chill the perishables. Make sure the milk and meat are right next to the cold packs. Keep the fresh produce away from the cold packs. Cheese doesn't care. Don't pre-prep anything that needs chilling, though. the quality will deteriorate and there's more chance of contamination,

You can probably get away with making the bechamel ahead, but it could break.

In the car, wrap the cooler in blankets or heavy towels.

We've travelled for weeks with commercially flash frozen meat as well as pre-cooked, home frozen meals, packed with dry ice, with no problems. The big thing is keep the cooler out of the sun, and don't open it unnecessarily,

My feeling is there's more air in pastitsio than lasagna. It should travel fine frozen, but give it every chance you can to keep it frozen solid (for quality as well as those pesky pathogens). Let steam out well while cooling after baking. Refrigerate over night, or equivalent time. Freeze it with plenty of air room around it, if you can, and also, if you can, put it right where the freezer coils are, or directly in the path of the fan (but not blocking it), so it'll freeze as fast as possible, with less water migration. Thaw and reheat at the same temperature and duration as it baked for (adjusting as needed for different oven, and elevation.

Good luck!

My preference is to assemble, freeze and bake later. I’ve done it many times with dishes like this. There’s usually a fresher taste to me than baking ahead and reheating.

If you use dry ice, be sure to keep the cooler closed and a window cracked. I have friends who nearly killed themselves traveling with a load of dry ice in their car. Their neighborhood had been hit with a power outage following a particularly bad storm. Being sweet people, they offered to drive to the nearest place selling dry ice and pick up a bunch for anyone who wanted some. On the way back, they realized they were getting very very sleepy. They put 2 and 2 together, rolled down the window, got some fresh air and were able to continue on their way with the windows open. Hate to be an alarmist, but I’m not sure I would have thought of the consequences.

I appreciate the responses. Since posting I’ve had a few thoughts and revelations:

-Everything, from pasta to meat to the bechamel is actually cooked before assembly, so i don’t think there are any raw ingredients to be concerned about.

-I looked at an ATK recipe and they say the assembled casserole can be refrigerated for two days before baking. Mine will be thoroughly frozen to -2°F until midday Thirs, then packed with ice packs. I could be wrong but I don’t foresee it being significantly thawed until sometime Friday. So if I start the clock Friday morning, it’ll be at low fridge temps for just about two days.

So I’m going with assembly only, and plenty of ice packs. No dry ice (interesting story/warning!) I am ordering some Re-Freez-R-Brix because they’ve got me curious. Sound pretty small but they might be good for my daughter’s lunches, plus my wife is making rumblings about taking lunch to work.

I think the freez-r-brix come in different sizes. Mine are each about the size of a pound of cheddar block. They're especially good where you want no wet.

If you pack your frozen dish with ice packs and fill the air space in the cooler (you can also chill or freeze some dry towels and fill the space (right out of the drawer works too, if need be), and you cover the cooler and protect it from the sun (coats and jackets work here too), your dish shouldn't be thawed that quickly (my caution above was for fails in chilling, not anticipated thaw), but if it is, I agree that the fully cooked, ingredients, held chilled, going into an oven to be fully cooked, thereby getting rid of any hitchhiking nasties that slipped in during assembly, should be fine, just as you said.

Another good option for lunches is the gel packs where the gel can be spead in the garden when they start leaking and the bag can be recycled.

Re dry ice, that was a large load, not in with the food in coolers. It's just carbon dioxide. Can't breathe it unless you're a plant, but the amount you'd put in a cooler evaoprating, even if it all got out, would be like an extra passenger or two exhaling. Cars have drafts to make sure they don't go hypoxic on family road trips. Just not enough for big boxes of CO2!

…where the gel can be spead in the garden when they start leaking…

Or refrozen and turned into custom sized packs with a foodsaver!