As long as you can install a 1/2" ball valve in it, as well as a way to filter the grain, any shape cooler would work as a mash tun...
IMO, going with anything smaller than a 10 gallon cooler, for 5 gallon batches, is too small. Even then, you could easily be pushing the mash tun beyond it's reach if you do something like a strong ale, or something with a high OG (without using DME)... I've had 8-9 gallons in a 10 gallon RubberMaid cooler mash tun before, when getting the grain up to sparge temp (you need to figure on that additional water too).
I do still have my 70qt Coleman Xtreme (green) cooler mash tun... I plan on keeping that around for when I want to do something really big, that my 1/2 bbl keg mash tun is just a little too small for. Although the capacity difference between the two is rather small (about 2 gallons of usable space, or ~8qts)...
A 7 gallon cooler could be a good HLT, once fitted with a ball valve... Especially if you get the cooler cheap, like that price...
