It’s kinda hard especially when we are eve to find soil. I never makes stall my own before but i know we need a lot of longshaft and a steel adze. But sometimes i see people building stall and close the entrance using berries tree. And i think, maybe how about building stall using berries tree itself? That way we get compose soil even faster.
But does sheep cannot get out from stall made from berries tree?
Sheep certainly can get out from berry bushes. The design with berry on the corner is specific. I’m sorry to say a full on berry bush pen wouldn’t work.
How about pondation of oven? I see a person made something with that (but he was trap and die)
You can do adobe, stone walls, newton block, oven but the rub is… make sure you have an exit. I once died in a pen because someone added wall on the corner where I was going to use home marker. I was trapped in there and starved to death. 
And if maple branches are scarce you could always do any of the above and just add the fence gate.