You can try a halfer meshmod by throwing some enkmat over the existing pinwheel without shaving off any of the other pins if you want to do a 'test run' prior to a full on commitment.
As far as adding a bubble plate goes, you might as well pick up one of these:
http://www.vertexaquaristik.com/prod...ies/in-180.php
Would be ten times easier than buying and machining the acrylic then cutting open the body to get it nestled in there.
*In the best interest of full disclosure, I should probably mention that I work for the distributor of BK's and Vertex...