Plant campfires around the area, and especially near places that are probable spawn points, like near nests.

Make sure the arena is surrounded by sheer walls. Easiest way on the mountain is to pick axe the side of the hill which disrupts the smooth slope. Make sure you see the cliff texture all the way around. You might need to get creative in spots, busting up rocks or building up walls. That will keep wolves and golems out. Makes sure it's not so tall that you have a hard time getting in and out in case Moder goes somewhere you didn't expect. Ask for a friendly local wolf to help you test your work before summoning Moder. If done correctly the wolf will run around outside, get mad and break one of the campfires I told you to put down earlier. No big deal, just put it back after you deal with the wolf you drew in.

It's harder to keep drakes out because not all of them come from nests. Try to figure out the area where they're coming from and put down a few more fires. Otherwise, just do your best to clear them before you start.