Just about anything with a PPM out "trainer port" will work. OpenTX can read most PPM signals. You can probably find a used something for almost nothing. The problem will be getting the cable right...3.5mm jacked audio cables are commonly used but sometimes only mono works and sometimes only stereo works and some have strange connectors. Its just a shielded wire in the end but it can be annoying.
If you want to try the Spektrum wireless route suggested above you can usually find MLP4DSM used for not much. You do have to make up the wireless receiver though. There's an old article about it somewhere on FliteTest, and probably a number of You Tubes. You need an RX with a PPM out which most genuine Spektrum brand RXs don't have. Its mostly the copyists like Orange and possibly Lemon that have it
If you want something a bit more "put together" and price is not an issue then get another Q-X7, one of these https://alofthobbies.com/linker.html and a X4R -SB. I don't have one but those that do are very enthusiastic.
Just FYI, I don't recommend FrSky's bluetooth approach for trainer links since there have been bad reports. It is useful for other reasons but apparently not for a trainer link...might be old info. Not really an option for you since it requires both TXs to have the bluetooth support which the Q-X7 doesn't.
The trainer cable on the frsky side is easy. It just uses ground and the tip of the 3.5 mm jack. You can either figure out which wire is attached to it, or just solder all the wires that aren't the ground together.
Flysky uses an S-video cable plug, you need to solder two wires to it, the ground to the shielding, and the other wire to one of the pins for ppm. I can post pictures of mine if needed.
Personally I like the wireless route. But it's a bit more soldering. Not complicated soldering, just a bit more of it
Orangerx receivers can be purchased for like $10 from hobbyking, and you'll need a small switch as well.