I'm familiar with the Blue bug boilers, they are awesome! I have a contact that has 3 pot's designed after the blue bugs that hold about 150lbs each. They sure make boiling a few hundred lbs of craw fish easy work!
First thing, be careful with the the frozen 2 liter bottles. They can and will leach a plastic flavor when you put them in hot water. So, you may want to rethink that idea.
If your main goal here is to reduce the amount of seasoning used and save a few bucks for your friend, I would suggest the following.
I don't know what type of seasoning you are using. The best bang for your buck is to use liquid seasoning like Zatarans Liquid Crab boil. Obviously the liquid will penetrate the crawfish faster than a dry seasoning mix will. So, you don't have to use as much per batch. If you are using a dry seasoning then just reduce the amount of dry mix you would normally use and add some of the liquid seasoning to compinsate.
Also, you don't have to add ice to the pot to cool the crawfish down to soak. I know it dose work great and makes an exelent final product. But some times you have to change your process to accomidate what you have to work with. The way I have done it in the past when dealing with a large amount of dad's is this. Bring your water to a boil and add your crawfish, crank the heat all the way up! As soon as the water comes to a rolling boil, kill the fire and let them sit in the water for about 7-8 minutes. Then you can dump them into an ice chest and butter them like you normally do. Of course you have to taste them to check the seasoning. If they need more spice you can add a little of your dry seasoning to the ice chest and shake them up. You don't have too add much of the dry seasoning once they are cooked. just a little will go a long way. After the first batch you can adjust the saesoning in your pot accordingly.
These are just some ideas that might help you out. If you are like me, you will hate the idea of changing your recipe and method for boiling dad's. But belive me, when I cook 50lbs at home for the family I do it my way. When I have to cook 2000lbs for an event, I do it the best way I can with the equipment I have to work with. I hope this helps!