Hey @pushkarnk,
In general classic confinement requires of publisher vetting, which has following objective:
To ensure that the publisher of a snap is genuinely associated with the upstream project or belongs to a trusted group (e.g., Snapcrafters, Canonical, Verified Accounts ).
Considering the provided information (upstream-relation: user) I think you don’t meet this criteria (please correct me). Thus, I think the best option to proceed here would be to ask the upstream if they are willing to incorporate the snap package to the upstream repository (or a different upstream maintained repository).
Thanks