Digital signatures are well supported so I wouldn't worry too much about it holding up in court as long as the agreement is valid per contract law and there is a clear for affirmation of the agreement.
A super easy and cheap tool is NitroPDF, which lets you digitally sign documents but it would be tricky to get your clients all to install it!
A really good system that is totally online (and super easy) is https://rightsignature.com/ - if you just have a single document that you need signed over and over you can squeeze into the personal plan which is like $15/month and works great.
The fewer our wants, the nearer we resemble the gods. — Socrates