You can do this by ordering small to medium FHM only. Large FHM are males, often old and likely nearing the end of their lifespans. Female FHM run much smaller than males, by ordering small/medium FHM you stack the odds in your favor significantly. Sure there will be plenty of males in the smalls, but they will have many more months and spawns in them than the large adult males. In the last 4 years I haven't stocked a FHM for a client larger than 2" as far as I can tell.
Interesting. FWIW maybe I did something at least a little bit right for a change. When I did my original FHM stocking I did it by going to the bait shop at the state park. They had small "crappie" and large "bass" size FHM. As it turns out a "dozen" to the clerk at the shop is a net full. So each time I got 3 "dozen" small and 2 "dozen" large I was really getting over a hundred fish. Due to their way of measuring a dozen, I probably stocked around 80% small as you can get a lot more small ones in the net than big ones.

I stocked a total of 25 "dozen" into my 1/4 acre and had tens of thousands after a few months.