Use a slab like the shape of your polygon and do a SEO (Solid Element Operation) cut, and then convert your morph element to morph element again. Works with AC22, and I believe with AC21 too, not absolutetly sure though. Please just go ahead and try it.
I use complex profiles for these, then cut them with SEO, and then convert them to morph. And I keep my complex profile design in a safe place in case I need to change the profile on use some other kind of polygon to do a SEO cut.
You could use just complex profiles, do a SEO cut, and hide your SEO layer. Sometimes, or at the very end of design, you might want to simplify those SEO's out of the equation though.
edit. ok, my example is the negative of your design, but surely you got it.
AC25, Rhino6/7+Grasshopper, TwinMotion • Mac Pro 6,1 E5-1650v2-3,5GHz/128GB/eGPU:6800XT/11.6.5 • HP Z4/Xeon W-2195/256GB/RX6800XT/W10ProWS