at macs in the cheap tool rack i bought a set of uni bits, cone shaped stepped bits. best bits ever all of a sudden they have been used for 6 years and still drill. they have drilled a lot of stainless steel.
I have hit rocks and cracked them but ive never had one stress crack and we beat the hell out of them. i believe there is a guy that sells a tank where you cut the top off the old tank and it sits in the belly pan. rust seems to be the biggest issue.