as my fueling ain't so good (Well when going round left-handers) due to my tank/pump setup i'm considering making a tank to sit in my tyre well rather than underneath. I've seen the Alloy ones and some of them are

.
could i use mild steel to make one? i presume i could but was wondering why yous use alloy, or is it just lighter, or more
any construction tips/ideas?
(i've got a sh!tload of things i want to fab up)