Easy to print upside down. Use M2.5 heat set inserts. I've got mine from https://cnckitchen.store/products/gewindeeinsatz-threaded-insert-m2-5-standard-100-stk-pcs
Get FreeCAD model here: https://codeberg.org/uwearzt/freecad/src/branch/main/lab/gridfinity/prisma.FCStd
The author marked this model as their own original creation.