We are planning to introduce three leather cases for the most typical HP calculators. We will start with:
The key for both the Voyagenr and Pioneer cases is that they fit quite tight - and do not slip out immediately. Normal fabric is ruled out, while suede- or felt-type may work well. Also, in my opinion, the suede-like feeling of the inner part of a leather may work, too.
We will keep you updated. Meanwhile, let me show you 4 different types of HP41c covers,
And also a Voyager Burgundy leather case, and a similar case for the Clamshell type: