Hi Everyone!

New to the forum and writing my first post here. I live in Acton Massachusetts and I have the intention to build a 42" this next spring / summer
My intention is to build a corner version and probably I'll connect with a few other elements to form an outdoor kitchen.

I've been reading many posts, I've read the PDF with the instructions, and I've been doing research for a while.
The time to get more serious with it has come, and as many of you, I have a ton of questions.

My first question: I think I want to pour the concrete top using a HardieBacker board, instead of wood. I've seen that people leave a few inches to the edge for the concrete to fill the wholes on the cinderblocks and reinforce the strength of the top. As the board will rest on the cinder blocks on the sides, do I still need to build support with 2x4's towards the center? or would the board be able to hold the weight of the concrete?
I haven't been able to find anything related to it.

Thanks in advance!