Great idea - certainly a good size for parts if you have the space to store it. Unfortunately I don't have that kind of space here. I'd recommend sorting out some sort of filter system on your old refrigerator if I were you - then you'll have a better chance of seeing what you are blasting.

The cone design is something that I wish my cabinet had - then I could just kick the sand (actually I'm using glass media) back towards the pick up tube instead of having to stop and shove it into position:- But this seems to be the only downside to my entry level cabinet that I've come across so far.

Good luck with your own design. I'd be interested to see some photographs of the finished product.