I don't use those plastic supports, I throw them away all the time.
The primary support for an acrylic bath is always anywhere it touches a wall. Batons secured to the wall are placed under the rims so the bath edge 'just about' sits on it when levelled. Then run a good bead of silicone along the baton where the bath edge will sit and another bead along the 'clean and dust free' wall where the side of the bath will touch it, paying more attention to the long and the short edges where the shower will run if there's to be one. Lift and push the bath onto the silicon, push and press down, there should be enough silicone applied to squeeze out all along contact edges, clean and smooth the silicon along the top edge to give and that's the first seal, 1 along the bottom and 1 along the wall. Check levels again. Fill the bath up to the overflow with water, and leave at least 12hrs.
The framing in the pic is extra to provide full support for the open 2 sides of the bath and the best you can provide. If the bath has 2 open edges, as if you place weight on the front or open corner, it will pull that area down and the unsupported edge(s) at right angles to it putting force upon the seal. If the bath has only the front long edge open then it's not needed so much as both bath short edges are support so there little or no corner flexing.