If understanding the OP correctly, i would not recommend drawstring packs for a front chest rig. I own several and the biggest limiter is that there is no form to the shoulder straps and the bag just hangs there. On my back I tend to use a D-ring as a sternum strap just to keep it in place. Wearing on the front the thing would just hang at your waist and try to slip off your shoulders.

I have an older version of REI Flash 18, packs super small and cinches tight when worn. Great assault pack.