Mash Tun Size

Sorry if this has been answered already somewhere but I've been looking through the threads and not really found what I'm looking for. I currently do 5 gallon batches batch sparging and using a 48qt cooler/mash tun I made. It works great and holds temps really well. I'm now interested in trying an occasional 10 gallon batch but mainly 5 gallon ones still. The most grains I'd currently use is about 29 lbs for a 10 gallon batch. I found a decent price on a 70 qt cooler but is that too big for holding mash temps without any kind of filler inside it if I mainly do 5 gallon batches. Thanks.

Why not simply keep the 48qt cooler that's working well for your 5 gallon batches and build out the 70qt into a tun geared toward big 10-gallon batches?