I'm in the process of making my own version. I am going to stick with the PC fan also, that inflator is too noisey. Also, you can find 3-speed PC fans to increase flow if you want. Instead of in the box, I'm going to have the fan at the end of the output
Instead of the pipe, I'm using flexible hose.
http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId=-1&ca
talogId=10053&productId=100119536&N=10000003+90072
And putting the small fan at the end of it. It should almost fit perfectly across it.
As a twist, tomorrow is going to be a very sunny day so I'm going to fill one of the black cans with engine oil. I want to see how hot the engine oil will get. If it gets very hot, it will be a great way to trap the heat for later in the evening when the sun is gone.
I'm also painting some aluminum flexi-tubing black to see how that works. Then instead of connecting cans, maybe making a bunch of lines of tubing instead. That way you don't have to mess with silicon and you have one tube from input to output.
http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId=-1&ca
talogId=10053&productId=100010697
The flexi-tubing is very cheap and really easy to work with, I'm kind of excited to see how that works.
Another alternative would be placing the flex-tube thru the cans which contain engine oil. So you would have the air surrounded in hot oil at the end of the day and could pull the unit inside and cycle the inside air through the hot oil.
Water gets to 110F with a decent solar setup, if oil got that warm it make be good for heat several hours after it is in the sun.