the safety and/or usability of making and using a sandblaster out of a short wide thinwall tank like a 20lb propane can be argued amongst yourselves.... I personally would not be within 50ft of one.......
some will be driven to construct one simply because they can.... not because it is a good idea.... and they will....

the real reason for this post is to suggest that if you plan on doing any sandblasting outside of a cabinet...
Wear Protection!!!

Silicosis is a very real hazard to people who sandblast... so make sure that you have a source of clean air under your hood while sandblasting...
here is a link..

http://www.silicosis.com/respirators/index.php

take some time and do an additional search, read and understand the hazards.. before the next time you sandblast...