The fans I use are made by Ice Cap .... sometimes I see them shown as "smart fans". I bought them online from Premium Aquatics, but I know many places sell them.
The 3" fans are around $40, but it has been well worth it because they are such time savers and have lasted really well. I had to replace one fairly quickly because the probe mafuntioned, but have not replaced the two I have for easily 2 years.
I'm not real creative with the DIY when it comes to stuff like this, but I have seen people rigging up computer fans to work in their canopies. You might do a search on RC for that. I just wonder how the temp probes designed for a computer last in a corrosive saltwater environment, but maybe they do fine.