Create Unblockable First Visit Popup

I have a website at [login to view URL] that is an investment site I am working on. Because it is a legal site, it's pretty sensitive information. I need to have a legal disclaimer pop up but only on the user's first visit only. The person just needs to have to click "accept" to the legal disclaimer than then it doesn't need to be seen againI'd prefer that it showed it every 30 days based on a cookie but once is okay too). The popup itself is just pure text that is a legal disclaimer. I don't need anything fancy except that it doesn't get blocked on most browswers. I need the project completed in 1-2 days. Because it's a small job, I will pay in full when the site is completed. The site is a wordpress theme so I just need the code to popup on the index page only.