I used Docusign.com. We had a PDF approval template and it was setup as a fill-in form that we could drop in a screenshot of the artwork, then upload to docusign and have the approval process managed through there. I would be able to see the entire history of the proof, including when it was approved, and it would send me a notification by email when it was approved. I also set an "expiration date" which was the latest that they could approve the proof for us to get the order completed when promised. If there was specific detail that I needed the client to see, I could upload individual screenshots at 100% zoom or whatever was needed and have them attached to the proof approval package. It worked great for us.
Granted, I was running a tradeshow and exhibit company with TIGHT deadlines, so if they missed the approval date/time for art, they were subjected to additional fees as stated in the contract they signed before the job started. Honestly, many of our clients didn't mind paying additional fees for graphics, because they were typically only 15-25% of the total cost of the booth. Especially the huge ones like you see at SGIA and ISA (several of which were my clients). They would spend 75-150K on a booth for custom fabrication and graphics.
