1. Download Meshmixer
http://www.meshmixer.com/download.html
2. Import .stl file, use Hollow function

3. Add through holes on you sample, adjust hold position. Always leave a through hole on hollow .stl for trapped resin.

4. Click accept and get you hollow sample!

Reason for hollow samples:
1. Large solid part can waste a lot resin, hollow part can save resin
2. Large solid part will have a large contact area, cause a large peeling force. This peeling force can cause some print failure. A hollow sample can improve print success.