I wondered about this as well. I've seen a lot of foundationless frames with space around three sides. Do the bees tend to fill this in over time? Or do you live with a lot of spaces?
I have a guess that when they get to an obstruction like the bottom bar they think it is the bottom of the hive and allow that space. I think this because if you use foundation they will completely fill it in. So if you let the bees draw the comb down to the bar and then close the space off my suspicion is they will fill it all in like foundation. If you put the Popsicle sticks in first it won't work because they will just draw comb within 1/4 inch of the Popsicle stick.
I suppose you could leave the bottom bar off and let the bees draw the comb down to the bottom of the frame below or bottom board and then add the bottom bar which may require trimming of the comb.
This is all guess work. Someone who is doing foundationless would have to prove or disprove the theory.
Zone 5a, Practicing non-intervention beekeeping
I am also using foundationless and as Ace, I also only have a few hives. Referencing other beeks (such as Michael Bush / Michael Palemer / "oldtimer" etc) you'll get much better info. When one of these guys post on a thread that I've started I'm thankful. They're generally humble and open, but know so much about beekeeping and willing to share it. I have seen one hive where the foundation was cut out on the corners to allow the bees the communication space. Michael Bush said on his site or on a thread somewhere in here that he used to buy deep foundation, cut it in half and use it in medium frames. It would give a couple inches at the bottom for them to fill in however they'd like. Now if you look at some pictures on MB's web site he has pictures that have frames attached in a couple spots to the bottom bar. Not the whole way, but a couple spots and once the comb is a little older he hasn't had problems extracting. IMO if he hasn't had issues then I'm not going to worry about it. When / if I get big enough I'll look at extracting at that point. For now I think it's cut and strain for me. Bottom line if others that have been there and done that with good success I'm not going to be concerned.
All my hives are foundationless and all the comb is only attached to the top bar with 1/4" gaps on both sides and the bottom. My bees tend to leave a tunnel about 1/2 inch down, dead center of each comb on each frame. Even though we have a month and a half of 100+ temps, I don't wire and have never had an issue with sagging comb. The most important thing is to make sure they draw the first frame straight. I would also suggest using 1 1/4" thick frames for brood in the brood chamber.
I run a lot of foundationless frames. On my deeps I put a 1/2" square piece of wood in the middle of the frame with the edges pointing up and down. I reduces the size of the comb to about the same as a shallow. I find the bees fill it pretty full, attach the bottom pretty well and the sides very well. I've had zero failures since using this method. A little more trouble but no more than installing foundation or wiring. I rip the 1/2 inch out free pallets I get down the street. I interweave with PF-100's in the brood nest and find the bees fill these much faster than the plastic.