I bought a camping pad from REI and it's worked great for my daughter. Something like this can be used again and again vs. the egg crate.
https://www.rei.com/product/187551/kelty-mistral-si-sleeping-pad
My daughter is at sleep away for a month so I can see that over 10 years it is a good expense.
Something else to think about, OP, is the actual size and/or age of the mattress - the egg crate might not fit, or the mattresses are old or newer (in which case you shouldn't need one). For example, my daughter's camp is more "army cot" (length is normal, but width is probably 2/3 of a twin) where my son's is normal twin size. Might be worth a call to camp to see.