So, you want to access some content from inside a page that you created from an existing master page? The most clean way to do this is to create a public property in your master page, then access that. In my case, I have a search button on a master page that I want to use from inside to pages that derive from that page. So, Here is how I declare my master page:
In my codebind for my masterpage, I have the following:
Then, in my page I derive from the masterpage I have the following. Notice that I include the masterpage type so I can have type safe access to the public property.
Then, finally, in the codebehind, this is how I access my search box. In my case, I assign it to an invisible label, then I can have server side presentation controls, like GridView, access it as a select parameter.
Good luck, and hope this helps!