I've tackled these dilemmas in my own ways. The advice previously mentioned is spot on! But there's a few ways to skin that cat. I've taken clones as soon as I see preflowering or just before from the lowest undeveloped branches. These can be reversed and pollinated. I pulled 40-50 Off a gorilla glue clone and it was only 5 inches tall. Another sister clone was reversed next to it in the same 1 gallon pot.
But to isolate pollenation in a single grow area I used a clear plastic tub w a latching lid . I cut an intake vent on the side of it and taped a piece of high quality air filter material over it , then repeat this at the highest point with an axial fan drawing air out. Melt a few holes for watering hoses made outta clear viynl hoses that will land in the pots of soil. Add plants after pollensacks form. Plug the fan in to create a negative pressure that keeps stray pollen inside. Then add to the grow area. As long as u pull it out to open it in another room and then spray it down w water after u close it before putting it back in the grow.
If u Google male cannabis isolation chamber u will see several examples. Total cost isn't much and it's super easy to do !!!!
Dope! Thanks for the input. I've seen a few isolation chambers online.. Im not to savy with diy tho. Lol. I may give this a try though because I really want to make a cross durring this run!