Well, don't worry too much about the feelings of the buck, he may be quite agitated when in a cage that is close (extra cage, or solid divider, not just a wire divider), but as far as I've seen they can deal with that. Anyway - you'll have to remove him in time, since the last days are quite a strain to the doe, most likely she will not mind being left alone (you'll have to make up with petting and nose rubs), I would remove the buck at least a week before kindling if it's a very nice, calm and friendly buck, if he pesters the doe I would remove him right away. This would be a good oportunity to have him neutered if you plan to put them together after she kindled (without planning another litter), by the time his 6 weeks of healing and getting infertile are over the kits will be old enough to leave the nest and slowly start on solid food. I've never seen an adult rabbit being anything else than friendly to kits.
If you're not absolutly sure about the date she's due it's better to remove him early. Although the stress of a litter is imho somewhat overemphasized in the first link, a second pregnancy right away is really a hard thing for most domestic rabbits (wild ones are smaller and have not so big litters), and prone to severe problems. Although there are some breed lines that can handle that, those are imho an excemption und not pet breeds - have that in mind if anyone says it's not a big problem.
The nestbox should be big enough for the doe to lie in and turn around, although most I've seen online use a open design with a lower front wall, I prefer a closed wooden box with removeable top (for checking, or keeping a gap to control temperature) , twice the size of the doe (lenght doe stretched out, twice her width), with an entrance hole 2-3" from the bottom with a ramp in front, so that kits can't get out too easyly but can go back without obstacle. Also, the doe keeps the entrance stuffed closed for the first two weeks (natural behaviour) and maintains a nice nest in that imitation of their tunnels they would use outdoors.
Since you didn't ask what to do with the young (you should have a solution for that 8-10 weeks after kindling, depending on breed, you'll have to seperate the bucks or you'll find yourself in the middle of a bunny explosion) I assume you already have plans for that.
