Newbie looking at 6040 (China CNC) for polypropylene 15mm sheet

[Zeeflyboy]

Still relying on your vice...

Make that piece (except minus those channels you added to allow it to flex inwards) out of something nice and sturdy - either alu or something like SRBP. Add a couple of threaded holes either side to allow clamps to hold it externally, and maybe two threaded holes internally underneath those cut outs. I would also add a zero reference hole (the probe works better determining zero from a hole probe than from an open edge probe), though I would ream it rather than just drill/mill it.

That then allows you to clamp internally with just a screw and spacer, run your external ops, then switch over to clamping externally with screws and a clamp piece to run your internal ops. Only takes a minute to undo two screws and screw in the other set for switching from internal to external.

depending on what you are doing operations wise, you can also consider the external clamping from both the ends and the sides to allow you to run different operations with external clamping but it doesn't sound like you'll need to.