Workholding for 2-sided acrylic

I’ve got some 1/4" red acrylic sheet that I want to do some two sided machining with. The shape I want to make will essentially look like a 3" x 6" shark’s fin. Does anyone have any suggestions on how to hold the work and register it for the flip? I’d consider modifying my dimensions and using the flip jig, but I’m not sure that would work well with thinner sheet material.

If I were doing the “fin” here’s how I’d go about it.

First I’d model the rawstock itself with holes for two screws that I would ultimately use for fixing the stock onto the spoilboard. The holes are on the horizontal centerline of the stock and outside the area the “fin” will occupy.

I’d import the stock model (STL or an extruded DXF) into MeshCAM and place some flat thin supports near the corners. Use 2.5D machining to cut out the rawstock (waterline + pencil finishing, do not machine top of stock, no roughing). I’m sketching the supports as the red rectangles (which is the way they’ll look in MeshCAM)

Since you have modeled the stock it is an accurate size, which is needed for 2-sided machining.

Use the actual dimensions to input the rawstock for your fin. Center the fin in the rawstock in all 3 axes. Set Program Zero to the left front top corner of the rawstock.

Drill some alignment holes into the spoilboard like I did for my Millenium Falcon attempt. There is some discussion at Can I use Nomad to cut dowel holes in spoil board?

Put dowels in the holes to align the now-accurate rawstock. Screw down the rawstock using the pre-drilled holes. I drill pilot holes in the spoilboard using a hand drill.

Zero your cutting bit on the front left top corner of the actual stock. Machine the front side, flip (the fixing screws will go into the same pilot holes since they are centered vertically on the rawstock) and machine the back side.

As @Randy said, pins work. As is the basis for the flip jig.

I actually prefer to cut a pocket into my spoil board to have an accurate alignment and work piece holding.
I did a 6 sided job here.
Since then, I bought one .75"x4’x8’ sheet of MDF and ripped it to make over 30 pieces of 8.5"x11" spoils.

There is not one correct way to do this.
The ONE thing that matters most for any multi sided job is accurate registration for sides 2+ from the original job.

It is worth noting that for any use of the spoilboard for alignment, it will only be true as drilled or machined until it is removed unless there are locating provisions in the spoilboard itself to make sure it is installed in the exame same location on the bed again. I have several spoilboard jigs and use the vise often, so I drilled out provisions to use the vise pins in the spoilboard for alignment.

@dr_g Good point on re-registration. I have been using mine as step zero when making a batch, and have not been re-attaching them after the batch was done. Time to get some shoulder bolts and cut registration holes on the bottoms of my spoils.