I'm thinking something like one of these
but I can extend that intake across the tank.
I would not recommend extending the intake. Instead, put the powerhead itself under your sponge in the compartment at the far right end in your diagram and use vinyl hose or PVC pipe on the output side to get the water to the other end of the tank. Besides being better for the pump (small centrifugal pumps really don't do well with restricted intakes) it means you can hide the ugly thing under the sponge where no one will see it, instead of having it hanging in plain site in the display portion of the tank.