Coming from a commercial art perspective, any commission work should
include both a prepayment- non-refundable- that covers the design
work you do initially, AND a basic agreement that specifies each
party’s rights and responsibilities. You can find prototype
agreements of this nature in books like “Business and Legal Forms for
Graphic Designers,” since that’s an area where prototyping etc. is
common practice. I also use “Legal Guide for the Visual Artist.”
If you’re doing work at someone else’s request, they should pay for
your time and effort (and materials). You retain rights to the work
until they buy them from you. The approval process should be
specified. Then, when there’s a design they like, you negotiate the
fulfillment contract, whether that’s you doing the work, you selling
them the design and rights to have others reproduce it, royalties, or
whatever combination meets both your needs. If you never agree on a
design, but you like the one(s) you’ve come up with, you should be
able to make 'em and sell them.
This is assuming that you’re doing work “inspired” by the museum’s
pieces. If they want copies, you need more up front, since you have
fewer options about using your work yourself if they don’t go for it.
I really recommend involving clients in the design process. They
usually want to have you complete the work, then decide if they want
it (and if they’ll pay for it), but that leaves you pretty
vulnerable. If they’re involved, like needing to approve sketches
before you make prototypes, a lot less time, effort, and materials
are wasted and the process is more fun and pleasant for everyone. If
they’ve never done this before, their preferences aren’t writ in
stone, and they should be amenable to you telling them, That’s not
how I can do that.
Good luck! Your work’s lovely, BTW!