The best alternatives to Voxel Builder are Vectary, Kaedim, and Responsive Pixel Art. If these 3 options don't work for you, we've listed a few more alternatives below.
VOXELCANVAS is Web base voxel modeling tool. All FREE OPEN SHARE creation platform. You can create Voxel model on the web. Through VOXELCANVAS, you can have a collaborative experience like a sandbox.
This tool cuts down the time to make 3D assets by constructing models in a new way. It makes voxel models that are symmetrical along the z-axis. It's a great way to quickly fill a 3D scene or use as a starting point in more complex modeling tools.