I think all you’d need to add is redir / /guacamole/ 308. This’ll return an HTTP 308 response with the Location header set to /guacamole/. 308 means “permanent redirect”.
It would be possible to rewrite URLs to prepend /guacamole/ all the time, but there may be unintended side effects for that depending on how the app serves its files. I think it’s probably best to go the simple route.