This is what I've decided to do to use flat brackets. I have two cabinets and a dishwasher space (which has a cabinet on either side) where I need to use the flat steel supports.
Here is how they line up from the end: 1-2-DW-3. (cabinet 1-cabinet 2-dishwasher space-cabinet 3 (where no countertop support needed)
On the first cabinet I added a 2x6 flat against the back of IKEA cabinet, using 3" SPAX screws from each side. I did the same on the dishwasher space. On the cabinet in between (second cabinet) I used a 2x8, so I could get two screws underneath the 2x6's in the first cabinet and dishwasher space, and then put the top screws in at an angle.
I am using three different brackets. On the corner, I am using the Island Support System by countertopbracket.com. http://www.countertopbracket.com/ProductDetails.asp?ProductCode=ISS
Next to that, in the first and second cabinets, I'm using the Hidden Island Support Bracket by originalgranitebracket.com. https://originalgranitebracket.com/collections/hidden-cabinet-brackets/products/hidden-island-support-bracket
I am adding a 1x4 to the front of the cabinet to hold the front part of that bracket. That reduces the drawer height by 3/4", but since I am using the Reinforced ventilated top rail, that already takes up 3/4" so I'm not losing anything with the added 1x4. http://www.ikea.com/us/en/catalog/products/60284581/
The dishwasher space presented a challenge, because I did not want to lower the dishwasher by 1/2" or 3/4" to use the other brackets. And the space was very tight along the sides, so I could not use a sidewall brace. https://originalgranitebracket.com/collections/hidden-cabinet-brackets/products/calli-side-wall-hidden-granite-bracket
So I opted to use L braces, mounted to the 2x6. https://originalgranitebracket.com/collections/hidden-cabinet-brackets/products/hidden-l-granite-bracket
In hindsight, I should have used a 2x8, since the L brace has its bottom hole at 5.25", which is only 1/4" from the edge of the 2x6. To add more support for the 2x6 (already held in by screws on each side), I am adding a heavy duty Simpson L angle bracket http://www.homedepot.com/p/Simpson-Strong-Tie-Z-MAX-2-in-x-6-in-12-Gauge-Galvanized-Medium-L-Angle-ML26Z/202071149 to each end of the 2x6, and screwing it in using #10 screws, with another 1x4 on the other side of the cabinets to help hold the screws. This may be overkill, but the IKEA cabinets are particleboard, and I have to hang two of these L braces on this one 2x6. Unlike the other brackets, which merely rest on the 2x6, these L brackets actually act as a lever, so it's important that they are well secured.
I cannot seem to attach any photos here, so I hope what I wrote is clear.
Q