There was a piece in Economist recently (or was it Scientific American?) about blimps waaaaaaay up there (considerably above 20K) and how the normal problem of pressure differential leading to baloon failure was being approached, and possibly resolved. The sketch in the article showed a cigar/lozenge shaped, twin hull looking thing.
The air currents up there (plasma currents) are not quite the problem one experiences at the lower altitudes.
Be interesting to see if that is one of the solution sets the Army tries for this program.