You can use any combination of glass and glassless insert in a De Vere 504 carrier as long as there are no protrusions that will stop the carrier closing. None of them will interfere with the masking blades.
In other words, use the glassless insert you'd normally use on the top on the bottom (& tape over the holes) if you are wanting to use AN glass on the top & glassless on the bottom.
Or just get a few spare glass inserts & use light opaque tape to make apertures exactly the size you need. If you are really bored, draw up some bottom half masks & get them laser cut from aluminium.
All of these have been day-to-day things for decades whether with De Vere or other enlargers that saw professional use.