Thanks Karl. I actually got it to rinse out my nose. :D
I made my polycarbonate piece a little too narrow. As you can probably see, it just barely covers the chambers. I made the mistake to assume that the head bolt holes were symmetrically placed around the piston. As a result, the fill hole is off for the block.
It worked anyway, but I had to force the fluid in to fill the cylinder completely. This caused some leaking past the piston rings. Not much, probably a cc or so.