Provide fresh treats in the area/coop/pen where you want them to be at night. Do this about an hour before they usually go to roost. Turkeys eat the most early in the morning, after coming down from roosting. Then afterwards they start their cleaning/featherkeeping/grooming. They eat somewhat throughout the rest of the day and then it picks back up before turning in for the evening. You can use that evening feed to your advantage.
Then, before its gets dusk, corral them in, don't be too aggressive about it. Just walk behind them going left or right, so they will go the opposite way in front of you. Be leisurley and comfortable about it. If you have a special word or whistle/noise for them use it. Turkeys process things with their eyes but also their hearing. If you're consistent they will see this as a routine. Almost a ritual for them. Do this every evening for 8 weeks, and then you can get to where you just go out there before dark and they should be all in their right places. If some are not, corral them in, so they know to get with the routine. Some will catch on faster than others. You should be able, after doing this and by, hopefully end of summer, be able to go out right before dusk and they all are good obiedient birds, comfortable where they are, because they know the routine. It's good to do this training now. Because the days get longer and you have more time to make sure they are where they need to be. By fall the sun goes down alot quicker and dusk happens faster. And more of a challenge to accomplish.
Another note: Make the area they will be at night comfortable and and safe. Check to make sure nothing is digging around, look for scratch marks or places where predators may be attempting to breech. This is key, because once a predator senses a meal or meals, he will come back until he knows he can't get what he wants. Or if he gets his target he will come back again and again.
You mentioned a partially opened area for them at night. If at all possible, just to be on the safe side, you make want to take some precautionary steps of making that area more enclosed and less acceptable to coyotes, foxes, bobcats, etc. Once turkeys are pretty big, tree climbing predators like raccoons and possums are not that big of an issue. But not far off the ground bigger less climbing animals might try them.
And if you do get them in this good routine of roosting where you want them, and they do well with it for a period of time, and then you see turkey(s) going rogue(not wanting to go to designated roost area) at dusk, then check the area, make sure it's safe. Sometimes a turkey will let you know its not feeling safe at night. And by not going in, this is there way of telling you.
One more thing: If a turkey gets used to roosting low(closer to the ground), then he will not want to roost much higher. He will be used to roosting low and usually stick with that (because they think they are safe, and also because low roosting is now their routine). Turkeys are really good flyers, but usually they only fly when they have to.