So in a web search for a DIY baby mobile, all I was able to find were handmade mobiles available for sale. In addition, many of the ones on the market feature elephants or fish or whatever ... but hung as if to be viewed from the side, not from the bottom!
Here is my $1 brain-enhancing, neuron-pruning, stimulating* baby mobile. The $1 was for the bubble blower ring that's the hub of the mobile. The rest is stuff we already had in the house.
*In truth I haven't checked, so I don't know if my baby's neurons are actually being pruned. But she's 7 weeks old, and will wake up in the middle of the night and entertain herself watching the mobile, and you can't ask for much more than that!