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.