Introduction
You can quite easily create a "Do It Yourself" OGN tracker from commonly available (and cheap) parts. What this DYI tracker will have "on board" is only limited by your budget time you want to spent and your imagination. Think whether or not you need a barometric pressure sensor, accelerometers, sd card port. igc logging, etc..) in your tracker(s).
What is common, however no matter what extra hardware you install they will all do the most important job - i.e. they will broadcast your position radio beacons using OGNTP protocol. They will as well relay positions of other aircraft. And the basic hardware will cost you just ~25$
Once you build your hardware you just install dyi-tracker firmware on it - and you're ready to go.
If you don't like too much soldering and want to build your tracker as quickly as possible, consider DYI OGN Tracker based on ESP-32. That seems to be an easiest options today. If you like playing with soldering iron a bit more (and want to spend even less) consider STM32 OGN DYI Tracker.
ESP-32 DYI OGN Tracker
The above information, although still correct, has become "old" as we have now modules available which contain all the parts ready and soldered, while the rest is "just software".
Have a look at the TTGO T-Beam modules for about 25-30 EUR price, the software described under the above link fits those.
Check as well HELTEC ASR6502 modules with GPS, they can as well be programmed to work as OGN-Trackerts.
STM32 DYI OGN Tracker
The above decriptions is the "oldest" DIY OGN-Tracker (but no the oldest OGN-Tracker ever) built from STM32 CPU board and RFM69 modules.