I am looking to build a rectangular style igloo cooler mash tun. I have a spare cooler that is 21"x11"x10" (10 gallon/40qts).
Is this large enough for 5 gallon batches?

I use a round orange gott cooler that is 10 gallons and it works fantastically. Your cooler is perfect for infusion mashing. Now get to brewing on it!

Yep

40 qts will be large enough for all but the biggest. I have a 48qt Cube, and have filled it to the brim for a 5 gallon batch of a 1.110 RIS. You should be able to squeeze in anything under 1.085.

Cool.
Thx for the info.
Sounds like most people use false bottoms for the round tuns and ss tube mesh for the rectangular.
Anyone have any luck just using a el grande grain bag?

I use a circular SS braid in my round 10 gal cooler, works great. I have used the bag before, but not much. One time I would give would to have a larger diameter bag than you do vessel. Otherwise the grain and bag sits off to one side and you have liquid on the other...

I used to use an el grande curtain... It was made of the polyester mesh that folks use for BIAB material. I used it to line my bottling bucket when I wrapped it in insulation and used it as a mash tun. Now that I have a cooler mashtun I use a SS braid. If you use fabric, it works much better if you have some sort of false bottom to create dead space under the material - I used a steamer basket.

PSSP false bottom in a 48 quart Ice Cube:


Here it is with 26.5# of grain, with a little room to spare:


I regularly brew 10 gallon batches with this. With a 40 quart cooler you should be fine for 5 gallon batches.

I think you need something to supplement the bag as alluded to, so you have some dead space. If you just throw the bag in the cooler I think you would be drawing disproportionately from the side where the drain plug is. A braided tube or slotted manifold are reasonable options too.