This case is designed to fit your Raspberry Pi 3 Model A+.
Outer dimensions: 68x59x15.4 mm
The case is designed to fit a 30x30x7 mm fan mounted like in the picture. Fans like that can be found fairly cheaply online.
I included a mount for 4040 or 2040 aluminum extrusions, which is supposed to be glued on the back. The small holes are for guidance to get a centered fit.
Other options without a fan and ventilation are included.