Very recently Bast released an app called Voxel Paint. Try this on https://www.apewebapps.com/voxel-paint/
Generally it is awesome, but here are my further suggestions.
- Shifting the model
Just in case you have placed the model in a wrong way, you can adjust the model placement using the shifting function.
- Drag-&-Fill tool
Just a simple tool for filling the gaps in an easier way.
- Curser tool
If you don't want accidentally editing your models...
Model facing a wrong direction? No worries! This will fix 'em!
Hopefully these tools will be helpful.
Thanks for reading!