Longest Extension Tube for EF lenses

I used to have the 600 F4 L IS Mk1 and currently use the 800 F5.6 L IS. I often need to reduce the minimum focus distance (it is 6 meters with the 800) so I bought a set of 3 cheapies off E Bay and they work perfectly. With a lens like yours (or mine) I would strongly suggest that you get ones with metal mounts but other tan that they are only tubes with some contacts so the brand doesn't really matter.
My extension tubes came as a set of 3 with lengths of 13mm, 21mm and 31mm. Frankly the 31mm tube is of little use as AF gets rather sketchy and the maximum focus distance gets rather short. The 13 and 21mm tubes are much more useful as the AF works much better and they will allow some reach as well as shaving those, critical, few centimeters off the minimum focus distance. The Canon ones offer no advantage over the cheap alternatives (I have used both) and cost 5+ times as much.

IMHO the only advantage the Canon ones add, is they are simply built tougher.
For most users the kenko etc, are just fine.

Over the years I have picked up the Canon ones on the used market to use with heavy 1D bodies hanging off the back of my large lenses. Noticeably less "wobble" compared to my otherwise perfectly decent set of Kenkos.

Maybe these days some of the 3rd party ones are made a little tougher?