I crush directly into large bins and attach my crusher with ratchet straps directly to the bin (no boards needed).
First try to positioning the crusher at one corner of the bin (if a manual crusher only (not a destemmer), the handle can extend outside the crusher corner), and maybe you will not need boards. If the crusher supports fit on the bin like that, secure them with ratchet straps in that position.
Else, for a 4x4 ft macrobin, I am not an engineer, and I don't know if one needs to worry about structural issues across at a maximum 4 ft span to hold up just a crusher and some grapes. If not, one may be able to just lay the boards flat, at 90 degrees to the crusher supports (not parallel to them), it is more stable than running the boards parallel to the crusher supports, and keeps the output from the crusher off the boards. Then use a ratchet strap to hold the crusher to the boards, and more to hold the boards to the bin.
If you want to set the boards on edge, then you should build a frame for the boards to hold them secure in that position. Again, position the crusher supports at 90 degrees to the longest board of the frame you build. Secure with ratchet straps as above.
Stems: If you do not have a destemmer, then the lowest cost solution is just to get a comfy chair, sit yourself down by the bin, and run your hands through the must, removing stems (and grapes from stems). It will take some time to do, but doable. And the must can be cold. So you may need to take some breaks in the process.
