I note that EOS shop sometimes sells extension tube sets. Some of the third party sets have rectangular apertures which would not block a 36x24mm exposure BUT you cannot fit Canon extenders into them. For this reason I would ask EOS staff to check this compatibility. Canon's extension tubes are fully circular and accept their extenders protruding lenses. My recommendation would be to buy Canon extension tubes if you want to increase the magnification of your macro (or ordinary) lenses (apart from the 180mm which is compatible with the extenders) although they do cost more.