What's the year make/model of the bike? Maybe it's possible to pull up the specs....

Actually, this thought just popped into my head. You could possibly still use panniers, and possibly your cooler (depending on it's size). Back when I tried panniers and discovered heel strike, I asked for advice and one of the products still sticks to my mind today.

The Nashbar ATB panniers mount a bit higher than most, which would help avoid heel strike. No fancy mounting hardware systems or waterproof, but does have a good reputation. Run two of them and you might be able to 'cradle' your cooler on top between them.