You don't need an adapter, just put in a smaller load. I've experimented with using the same amount I'd put in the LB trench, which barely covers the bottom of the oven. This works just fine even if you don't pack it. I'm really not convinced packing a small load helps much, but it's difficult to know for sure. It doesn't last anywhere near as long as in the LB (half as many hits or fewer) but I was surprised that the flavour seemed to be better than full or packed loads. I need to experiment more with this.
Putting something in the oven, even just replacing the "screen" with a real screen, is a bad idea in my opinion. The Pax was designed to work with that thin solid plate, not a mesh screen. Anything else can cause hot spots where you don't need them and result in heat distribution that the Pax wasn't designed for. This could result in anything from burnt spots in your load to failure to heat up enough to extra strain on the battery that could cause premature failure.