Duct tape would not be safe and really shouldn't be used,while cable ties/zip ties are fine in most cases
Since the OP already has the cage buying it isn't applicable,meanwhile I will say a simple purchase of an Alaska to extend(remove panel as mentioned previously)would be a great idea and would make it much more suitable than just the cage on its own as it isn't that great of a cage size on its own for a Syrian.
The only cost to modify it would be the Alaska,the cost of mesh(required regardless),zip ties(again,no matter what they're necessary,so the Alaska isn't adding anything but the initial cost it has),and wire cutters(which can be used to cut the mesh as well as modify the Alaska and the cage itself to make the absolute best modification rather than merely meshing alone).It really isn't that bad,considering the floor space that would be achieved and the minimal cost since one cage is already just lying around!