Or buffet tray holders, if you like; "pan" and "tray" both seem to be used with the same meaning.

As well as various other things, "seno" can mean a concave space: a hollow, recess or cavity. In fact it's the first meaning in the DLE:

"seno
1. m. Concavidad o hueco."
https://dle.rae.es/?id=XZnzy5h

The most common use of the word in that sense is for a sink: a double sink is "un fregadero de dos senos" ( https://dictionary.reverso.net/spanish-english/seno ), and you can see it used here:
https://www.ikea.com/es/es/productos/productos-cocina/fregad...

Well, I think your "senos de bufé" are essentially these: spaces into which the trays or pans are inserted in the buffet server, as you yourself suggested. I don't think they are the actually trays, which are removable, but rather the compartments into which they fit. "Compartments" is actually a plausible term, and maybe it could be used here, but from my googling it seems that "compartments" in buffet servers are themselves removable food containers. However, I have found a few references to "pan holders", and I think perhaps this would work for you. Here's a "grand buffet server", which has a "tray base", two "buffet pan holders", two "half-size chafing pans" and "three third-size chafing pans".

As you can see from the picture, the five chafing pans fit into two pan holders (2 + 3), and I think the latter are fixed to the base. So these would be the "senos"; they're kind of similar to sinks, in a way.
https://www.surlatable.com/product/PRO-566679/Grand Buffet S...

Despite being called "grand", this item, for home use, is of course much smaller than the kind of buffet server they would use in a hotel, but perhaps the term is valid for your context.