A GPS project I'm working on is a 10 MHz GPS-disciplined oscillator combined with an ovenized OCXO. It is from instructables.com by author yannick99.
https://www.instructables.com/GPSDO-YT-10-Mhz-Lcd-2x16-With-LED/It consists of a PC board (artwork provided, as well as gerbers) and is based on an Atmega 328P (HEX and source code provided), an inexpensive GPS module, an LCD and the OCXO. It has been a popular project, and the author continues to update it. The only significant cost outlays are the ubox Neo-6M module GPS and the OCXO. The GPS module built up on a PC board, with antenna, is under $10 on eBay. I bought the surplus Isotemp OCXO model from the article on eBay for less than $30, I see there is currently one listed there for $14.98 from China.
There are several examples of builds by others shown in the instructable.
My build was interrupted by having to move out of my house for 6 months due to a tree fall; we just moved back in, and I am currently reassembling my lab and ham station in the basement.
Ted, KX4OM