There are plenty of good ones out there, I'll leave that to the other squrare users to comment on...
Just thought I'd comment on capacity.
You mention 30 lbs of grain for a 15 gallon batch.
I have been known to use 75 pounds of grain on a 20 gallon batch, (after strike water, fills the 30 gallon MT to the brim) so personally I'd be looking at more like a 60 pound capacity as a minimum.
Following that math, my 120 litre capacity MT holds 75 lbs of grain so 60 lbs of grain should require about 100 litre capacity cooler...presuming you don't want to do a higher gravity beer than my AGDTDiePA, in which case you should consider 120 litre capacity since you could then do 15 gallons of probably a 1.100 beer.
Oversizing on the rectangular coolers might be an issue too, I don't know and will leave that for others to comment on as well.