Try buying a feeding ring. They come in many sizes and can be suction-cupped to the glass so that they stay in one place. You may also use some one-eight inch tubing with a "T" connector and suction cup.

eta:

http://www.drsfostersmith.com/Produ...pcatid=6280&Nty=1&Ntx=mode+matchallpartial&Ntk=All&N=2004&Ntt=feeding ring&Np=1

 

maybe some fishing line across the top

 

here is a great DIY method;

take some airline tubing and one airline tubing coupler.



say you wanted a square foot of floating space, so you
cut about 38" of tubing, join them with a coupler so it's
airtight, then the whole thing will float! now tie any part
of the ring you made to a suction cup or filter in/outlet
with fishing wire, leave a few inches of extra slack, and
it will all stay in one place, while still floating with your
water level when you are ready to do a water change.

here is a fabricated variation on this idea;

 

Great idea spypet!

 

Solid. Great ideas guys, thanks