I don't think they'd be worth building for lunging... They might help make things go easier the first day, but after that, would be not needed.
I want a round pen like what I described in case anyone ever climbs on my pony, or any other unbroke horse. I want a round pen then, just to limit the head of steam they can get up. ;~D
I watched my son break his horse. This horse had not had anything done with him at age two when Charlie got him.
Charlie took him to the ranch he hangs out at and Joe, his friend took him thru the steps of breaking. When it came time to get on the first time Joe had him do it in a 12X12 square stall. The sides of the stall were solid plywood high enough that the horse couldn't try to get over. We sent a horse to a trainer years ago and he started them the same way.
The thinking is, in a small square pen they can't get going so fast, and you can get them stopped easier by getting their heads in a corner. HOrses can get going pretty good in even a small round pen.
The initial times on a horse is to just get them use to having someone one them, starting, not necessarily going, and most importantly stopping.
Anyway, just another way of doing somethng. It made me nervous seeing Charlie on a horse in such tight quarters. BUt after seeing it done, I think the horse even thinks it;s to tight to do anything much:). Of course getting on should not be done till a tremendous amount of ground work has been mastered.
BEcky