I have a few 3" endmills that are 3/8" diameter with 1/4" shanks - you get them from ice carving supply places. If you can flip the job and built a drop table or some middle-free build you’d be able to mill the top side, flip and mill the bottom and then would only have like an inch or two of foam to hand shape to match top to bottom. Far easier build, less engineering, but trickier to run - flip jobs take some head-scratching until you’ve dialled in your machine (and your brain).
Have a look at my LR32 build. There are a few recent builds also that use just side rails, no “bed”.