On another note, when I first saw your skull I was immediately reminded of the Grateful Dead.
From many of the photographs you have posted, it seems as if you enjoy creating one-of-kind WP in 3D. (And you're building quite a collection, at that.) It struck me, though, that you also might make a few extra bucks with skulls featuring Grateful Dead artwork. (See images posted below.) I could be wrong, however, I'd be willing to wager you could make a pretty penny on eBay selling one or two a week. They are unique, and I believe there are a lot of Dead aficionados out there who would appreciate these - and would be more than willing to pay for them.
I realize that you would be, well, not mass producing, but more like "mini" producing them - and this can quickly grow tedious and uninspiring. You know, more like a task, rather than an exercise in creativity. However, if you used a multitude of designs, this might possibly alleviate some of the boredom that might come from "painting yet anther skull." By the time you ran through your list of designs, though, and circled back to the first, sufficient time would have passed to renew your creative energies. And besides, you could always break the cycle with WP.
The first thing that sprang to mind is as follows. The name of the album is "Steal Your Face." The design seems simple enough.
Second is the rose from "American Beauty." Like the skull from "Steal Your Face," paint the rose directly atop the model skull.
A similar red rose (this time sporting an eyeball) was used again on the cover of "Winterland 73."
And then there are always the dancing bears. Either in a group...
or individually.
This is just a suggestion, of course, and any final decisions are left up to you. Personally, though, I would try one and see what happens. If you pulled a "Matt Stock" (or even myself, having done the same) and listed only one per week on eBay, you might be surprised at what you fetch.
Two years ago or so, I made a small series of ("turn-of-the-century" and appropriately "aged") "patent medicine" (or "snake oil") bottles based around an "Alice in Wonderland" theme. (I'll post a photograph later.) It all began with one small bottle, the "Cheshire Cat's Elixir of Invisibility" and, consequently, any proceeds benefited actual cats "in need." Only an inch and a half tall, these would sell for $5 - 10. It was, I believe, the "Mad Hatter's Mercurial Tonic" and "Caterpillar's Persian Opium" bottles that brought in the big bucks, especially the latter (even though the caterpillar is surrounded by tobacco plants in the Tenniel illustrations.)
I also made one or two "Squonk Tears" bottles (based on the song "Squonk" from ""Trick of the Tail" by Genesis.) Again, these sold surprisingly well. (I still have yet to try King Crimson's "Lark's Tongue in Aspic.)
So you might give it a try with some GD skulls. It couldn't hurt, and if it garners any supplemental income, hey!
Personal note. Of course, it is entirely up to you, however I would recommend "aging" the model skull a bit (thereby making it less "plastic-y) by maybe painting it an off white first and then lightly "washing" it with a few diluted grays, tans, and browns. If possible, I would also try to find skulls that have a removable jaw. This way the jaw can be removed, and the skull would be easier to stand up on its upper front teeth for display purposes. (Like the way the skulls are shown in the illustrations above.)