Different cameras weigh different amounts. 1.4x and 2x TCs shift that balance point.

Start with a simple flat "camera" plate - I started with the Wimberley P-5. It'll be versatile on anything, although it may not offer enough length for balancing. Then go with an L-plate for your camera. If you get a lens with a tripod foot, get a plate made for a tripod foot (these usually have a lip at one end to help ensure it stays straight. Regardless, make sure your plates and clamps will offer you enough slide room while still providing a safety aspect: RRS L-plates offer cavities to catch a Markins pin (that's what I use now), but don't offer any screws that'll keep a camera safe in a Wimberley clamp. Therefore, my early thought of putting a Wimberley clamp on my next Markins head is out the window so my camera isn't splat on the floor.