We are planning to introduce three leather cases for the most typical HP calculators. We will start with:
The ubiquitous HP41 case. We will use the exact same shape as the one we know. We are using as template the original one, loaded with a 41c+card reader, to make sure it fits. We will have to find also the yellow sponge for those that do not have a card reader.
A Voyager case (hp12c, hp10c, hp11c, hp15c, hp16c). The original case was in vinyl, but later versions were in PE and had a very bad behaviour - including the anniversary versions of 12c and 15c. There have been previously leather cases by HP very well made, with a burgundy leather external and a cream leather internal, with upper cover; but it was so nice and thick that it could not really be used for your jacket pocket - which is for me the original intention of the Voyager line. (also the later HP12c Platinum units were far too thick for jacket use)
A Pioneer case. This is also a thin calculator that can fit in a jacket pocket. I don't know who was in charge of selecting the cases for the latest HP17bII+ - again, they won't fit comfortably on the jacket pocket.
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: