OP is yet another poster from optionetics. I don't have anything against them, just noticing several people from the boards there are also here.
Neither the CME nor CBOT offers FLEX options on equities, so it's moot with respect to the topic at hand. I thought the OP was asking how to synthesize a mid-strike straddle. FLEX is simply OTC functionality with exchange clearing. I liked your ORCtrader post.
A ratio of calls and puts at the closest strikes combined to make a an intially delta neutral combination? Wouldn't that act close enough to a straddle formed at a strike when the stock is at that strike?
No. Any movement will cause the greeks to drift away from the straddle curvature. The result will be an increase in gamma/dgamma risk.