Good start, just some tips:
For the wall it looks like their orientation is set to vertical. This means even in the corners they will be straight up and not follow the rotation banking of the track. This can cause unintended consequences when you or an AI hits the wall causing bad wrecks as the wall is not following the same banking degree of the track surface. Essentially set to vertical acts like a ramp instead lol. To fix this set all your walls and fences to perpendicular orientation. You'll see right away in the sandbox preview mod the wall now bank with the turns looking more natural like track do in real life with walls. Simply select the wall sections and change them to perpendicular, option is in the W Section Properties window:
Also for walls, not sure if your track will be a modern one but you can add in softwall safer barrier real easy. It is just an extra W section you place in the front of the wall and set it to Amco Right or Left. This will act as a safer barrier and not damage the car as badly as the concrete walls would. You can reference other tracks that have softwalls to see how the top and side textures are placed. As for the wall and softwall height/width I've found the sweet spot that I personally like is these settings:
Walls:
Softwalls:
End result with my custom textures applied: