Glowforge isn’t manufacturing their own acrylic. You will easily be able to find what you need at another supplier.
For the pieces you have shown you would find good success with 3M 9502 tape. It is a double-sided tape that you apply to the underside of the pieces you are cutting. It gets cut through with the acrylic, then you peel off the masking and stick it down. It is not optically clear but in this application whatever kind of shadow it might cast would be even across the entire piece. I bet it would look good.
You can see some more adhesive notes of mine here.