Indication a big I.D OR O.D

Hello everyone
so I am trying to see what or were to buy an attachment for a Indicator.
I have these big I.D and O.D to indicate on a mill but my indicator doesn't reach
can someone guide in the right direction on how you guys are indicating something as bid as a 25 inch I.D

Thank you

I built a plug that goes in a collet, cross drilled for a rod that the indicator attaches to. The rod can be any length needed. Use a set screw in the end of the plug to hold the rod in position.