Any bright ideas... need to cut plexi in tight corners

[gregghia]

Need help... what tool can I use to cut this plexi? Its a 20 gal long. There are two partitions in there. The problem is the top has nicely cut small diameter holes for water flow. I need to create a row near the middle or bottom to create better flow.
My tanks are in place and plumbed. I can't get drill in the tight space, i tried dremel but it wants to go all over the place.... any bright ideas short of taking the plexi out and having to reglue?

 

[posiden]

What bit did you use? I would try to use a dremel with the cut off wheel on it. Just be careful not to let the hot plexi fling on you.