One suggestion is:
1 make a poly cube
2 scale cube to your sponge size
3 make good UVs
4 Create a celluar texture map that looks good
-as a texture map in After effects using the "Effects/Render/Cell Pattern" and adjust with a gaus blur and then a levels effect to taste
or
-in Maya using the dev kit "cellShader.mll" plug-in, bake your texture and adjust in PhotoShop, blur, invert, add smaller pores...
5 Use your texture maps as a displacement map
or
A more accurate Modeling method...
5 Create a "Smooth proxy" of your cube
6 Turn on hardware texture view and trace your texture map holes to make simple 4 or 6 sided poly faces
6 Poly Extrude hole faces 3 times;
- first extrude is close to edge
- second extrude in mid depth
- third extrude is scaled down and at the bottom of the hole
Then make a nice texture map with small pore details as bumps or displacements