The only position(s) which satisfy the OP requirement are what Profitaker alluded to: conversions, boxes or rolls. Long the box at a large discount to the strike-differential over carry can solve for 500bp in vol-edge. Approximately $.40 on a 5-strike, 30d box and $.12 on a conversion, net of carry. Large premiums on short box/rolls. The only positions flat greeks along the distribution, excluding rho.