I remixed this model to have smaller shells, so you'll need 4 parts instead of 8.
The original model was made to fit the parts together with 3mm filament.

I scaled it down to 62.5%.
With this version, you can fit the parts with 1.75mm filament.

Also available to purchase on my shop:
http://super3dworld.conceptify.fr/en/produit/koopa-shell/

Sketchup file on sale on:
https://cults3d.com/en/creations/smaller-koopa-shells