Although I don't have a TT bike (yet) I do Eddy Merckx style TT's on my road bike. Good position involves keeping the back as flat as possible which does put strain on the neck.
I do neck isometrics to strengthen my neck during twice weekly yoga sessions. These sessions involve plenty of planking which also works the neck. This seems to work
pretty well.
Here's a link to a page with neck isometrics:
https://www.baycare.org/health-libra...at%205%20times.