How to delete the cutoff edge on importing terrain

[BattleDogDude (Astronomite)]

I’m using the terrain importer for heightmaps. I’m trying to make all this terrain tapering the edges disappear as it will be unseen and is unnecessary. How would I do this
Photo for reference:

I already have the insides gutted,

but the edges are still there. I’ve tried both making the unused area all black and all transparent but I got the same results. Here are the heightmaps.

[BattleDogDude (Astronomite)]

Update: I have fixed the issue.

If anyone is curious about how I fixed it, instead of using heightmaps to cut off the terrain, I used the air voxel for colormaps instead. This cleared out and cut off the terrain. I ended up removing about 1/3 of the voxels present, which I consider a success.