I think some of the SP maps can make very interesting siege maps.
the first mission t1_surpise the one with chewy in the desert i think would make a great siege map since it's a pretty small area. T1_rail is interesting too. the crimelord one in t2 (i think duo is trying to recreate?) , and t2_wedge(the one you are trying to set up explosives for xwing pilot) are really good , just gotta add some objectives to them and tweak a bit.