I'm just having a similar issue with a podcast I just uploaded on Castopod (which supports ActivityPub).

My first thought was creating a subdomain with the name of the podcast mordiscos.eapl.me

Then I watched that the software allows many podcasts in the same domain, so I had to pick a handle: https://mordiscos.eapl.me/@podcast

So now I have @podcast@mordiscos.eapl.me when this one is 'more correct' @mordiscos@podcast.eapl.me or it could even be @mordiscos.eapl.me I wasn't aware of all that when I setup Castopod (documentation might improve a lot, IMO)

My point here is that it's something important to think from the start, otherwise is painful to change if it's already being used like that.