I have NOT used it, but I have seen many positive comments about using Verifyle for a secure portal, and that seemingly includes e-signatures (but I have NOT checked on that).  In the event you are a member of NAEA, I have heard that Verifyle is free.  But again, I have no first-hand experience with that; that is only what I've read from other people's comments.

https://verifyle.com/pricing.html

https://www.naea.org/member-center/verifyle-pro/