20x8 strips veroboard:

Board is fully cut under the 741.

For a 10k@25°C RTC:
Given value for R1 (8.1k) switches fan on at ~31°C.
The lower R1, the higher threshold temperature.
R3 follows the RTC's middle-value.
IRF630 Mosfet suits all DC fans while beaing cheap.

Given value for R5 gives a 3.7v output with a 9v input.
The higher R5, the higher voltage.

D1 can be any >100V diode.
LM317 can be found in a TO92 package.