Mash tun capacity question

For Christmas, I asked for and received a cooler that I hope to convert into a mash tun for my foray into all-grain brewing. Anyway, the cooler I received is the Coleman 70-Quart Xtreme Cooler.

I got it and I'm thinking that it is WAY too big for me. I'm currently brewing 5 gallon batches.

Should I not use this for my 5 gallon batches? Is there some modification that I can do to make it accommodate smaller batch sizes? Any help is appreciated.

Look no further: https://www.homebrewtalk.com/f11/how-big-your-mash-tun-needs-123585/

So, it looks like I might be able to get away with the 70 quart as long as I batch sparge. I may also want to get a piece of foam and cut it to fit the cooler to reduce that massive head space. Is that about right?

Foam and/or some bubble packing to keep the heat in the mash. Lay some bubble wrap on top of the grain, then foam on top of that. Should help a lot.

Yeah, without some way to eat up some of the volume in your MLT, a 9 lb grain bill for a 5 gallon batch will yield sporadic temperatures.

FWIW, I have a 52qt Coleman Xtreme MLT, and with smaller grain bills and 5 gallon batches, I find that my grain bed has hot and cold spots, that never seem to equalize.

I can only hypothesize that the larger amount of headspace makes it difficult for temperatures to stabilize.

I normally brew 10 gallon batches, and have no problems with temperature fluctuations, but smaller grain bills with 5 gallon batches are problematic.

Finding a way to use foam or some sort of insulation to reduce the volume of headspace would likely solve that problem, however, I have yet to try it....I seldom brew 5 gallon batches any longer.

Is there a particular type of foam that I should use? Or would any rigid foam insulation work?

The pink or blue foam from HD or Lowes should work fine. Perhaps the sealed type with the plastic layer on it to help keep the flakes out of your mash.

I thought of doing the same thing and wrapping the foam with cellophane.