This incubator is easy to build and should incubate your eggs at a rate of about 1 km per hour, give or take!
Check out the instructions on how to build it here: https://www.instructables.com/id/Pokemon-GO-Egg-Incubator/
This fan would work perfectly:
https://www.amazon.com/gp/product/B00002ND67/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=B00002ND67&linkCode=as2&tag=annycurra-20&linkId=39572d4c41d395d39b4c8958759908ee
