I second the Stabilized Rice Bran (SRB), start them off at 1/2 pound a feeding work up to a pound a feeding over the first week. It will put a smooth layer of cover on one. I have not had it effect one yet as far as eating or upsetting their digestion track. Along with adding fat it also helps with the hair as far as adding a sheen and healthy hide.
It will cost you about $75 for the 2 months feeding SRB in the meal form, $120 pellet
$170 for the chubby