The J-Cell battery, aka 7K67 or 4LR61 or 539, is a 6V alkaline battery, typically classified as a "medical device" battery. It is becoming harder and harder to find these in stores, since most things these days simply use Lithium coin cells. Its still easy to find them online, but if you need one fast, thats not an option. They also have a problem of not lasting well while in their package (I've had a few leak electrolytes and be dead a few months after stocking up, still before their "use by" date). I have an older SCUBA dive computer that happens to use these things, and its frustrating to try and hunt some down before a trip.
To remedy this, I have built an adapter that allows you to use 2 cr2032 lithium coin cells in a device needing the J-cell. Note that this might not be a valid solution for all applications of the J-cell, as I did not compare drain characteristics of it vs the coin cells (max amperage, capacity, etc) but it works fine for my use case. My dive computer (later revision of it) was actually sold with a plastic insert doing much the same thing. This part is impossible to find, however. Note also that for something as important as a Dive computer you shouldn't rely solely on this (have a backup, a buddy, know how to do without it safely!). This guide shows how I made mine, use at your own risk. Even if made exactly to spec things could go wrong.
Materials: Wood, small screws, copper
Tools: Saw, Screw driver, drill, sand paper, file, pliers. Optional: tin snips, pipe cutter
For the wood, I used a small piece of 2x1 left from another project.
For the copper strips: cut ~1/4" wide strips, one about 1.5", one about 2". This can be done by cutting 1/4" off a piece of copper pipe 1/2" and 3/4" respectively, or cutting a 2" section off a smaller pipe, flattening it, then cutting the strips from the resulting plate.
The screws should be small wood screws, 1/4"-1/2" with flat heads.
Trace a battery onto the wood block. Cut the wood into the rough shape of the battery.
Clamp across the grain while drilling to avoid splitting it. Using a 3/4" bit, drill a hole in the center of the wood. If you have to cut the block to thin it, drill first, then cut as this will reduce the chance of it splitting.
File and sand smooth.
Cut strips of copper for the contacts, one strip ~1.5", the other 2". The shorter one will be made to be easily removable so the coin cells can be changed. Place the strips one at a time on the wood cut out and bend the end so that it lays flat on the edge against the wood, one on the angled edge, one on the other flat edge to form the contact points of the battery. Bend a small divot into the middle of the strip over the battery hole. Trim the ends of the strips so they do not extend past the dimensions of the Jcell.
Drill a hole through the strips for the screws. Use a bit just smaller than the screw head since these must be as flush to the strips as possible. One strip (the longer fixed one) gets one screw on each side of the battery hole. The other gets a single screw on the edge forming the battery contact. Using a much larger bit, drill the hole a little larger to taper it so the screw heads will fit more flush.
Test fit the strips to the wood. File grooves in the wood to get the strips to fit as flush as possible. Once things look good, drill small pilot holes using the holes in the strips as a template. Cut two of the screws short so the ends will not poke through the bottom and secure the fixed strip to the wood. Use nail polish to cover all but the tip of the divot to prevent shorting the battery at its edge.If needed, remove and re-bend the strip to better fit according to how it behaves when screwed down. Use a full-length screw for the removable contact since only the one screw holds it in place. If it all fits, try fitting it to your device. Remove and refine the shape until it fits with the battery cover in place. A snug fit is good, as it will help hold this together and be less likely to fall out. Take off the removable strip and stack two coin cells in the same direction (ie: + on one lays against - of the other) and place them + side down (+ goes to the angled contact) and re-attach the strip. Test the voltage at the contacts, should read 6V. Thats all there is to it!