I have the same problem during lockdown. I put in a "butter" tub with two sponges, fill the reserve but can never get it above 60 and then yes, slowly goes down a couple every 12hrs or so. Although, my last (first) batch hatched out okay, some reason I feel I need to still get it jacked up a little higher.
I have just now added another tub of water and will see what that will do.
I bought the incubator used and it works great but it did not come with any plugs.
Thinking I should find something to plug the holes, what do you think?