@Nick Sayer created this GPS disciplined high precision oscillator to generate a very accurate 10 MHz square wave:

The basic design of the project centers around either aVCTCXO – a Voltage Controlled Temperature Compensated Crystal Oscillator or a VCOCXO – a Voltage Controlled Oven Controlled Crystal Oscillator.

nsayer shared the newest version of the board on OSH Park:

Geppetto Electronics sells the module on Tindie:

A GPS disciplined 10 MHz frequency standard – ±1 ppb or ±0.1 ppb