This is my design for a new style of battery pack very similar to the existing packs but with a screw-cap made of some conveniently sized plumbing parts.
Due to the fact I'm going offshore tomorrow I haven't been able to actually pressure test this as the epoxy is still curing but I'm confident it'll work.
The following parts were sourced in B&Q in the UK. The measurements given are based on using those exact parts you will have to adapt for the parts your local DIY store carries.
2 - 35mm Circular Discs (Perspex, ABS, metal whatever). I laser cut mine from 3mm acrylic.
2 - 40mm X 32mm ABS Reducer - [link]
2 - 32mm Coupling - [link]
2 - 32mm ABS Access Plug - [link]
32mm ABS Pipe (32mm ID, 36mm OD) to make 2 Battery tubes 194mm long - [link]
2 - Button contacts
2 - Spring contacts
The little laser cut acrylic discs for putting the battery contacts on
First up read how to build the existing battery packs as the basics are the same.
Now lets begin with the traditional "MS Paint" rendition of what we're trying to achieve.
Put a 35mm Circular Discs into the bottom of a 40mmX32mm Reducer and Plastic Weld/Hot Glue into place to form a "Cap".
Drill a small hole in the bottom of the Cap and push two 18g wires through.
Rough up the inside of the Cap with sandpaper to give the epoxy something to key to.
Fit one wire with a Button Contact and one with a Spring Contact.
Measure the distance from the Button connector to the bottom of the Spring connector as 205mm (the length of three Trustfire batteries).
Hot Glue the longer wire (the Spring Contact) to the shorter wire (the Button Contact) where they both go through the notch on the acrylic disc the Button contact is glued onto.
Strip the wires a little where they will be in the epoxy to prevent "water jacket" effect. Make sure you "splay" the wires a bit so they don't touch in the epoxy and form a short circuit when the epoxy has set.
**DO A DRY RUN FIRST TO MAKE SURE YOUR TUBE LENGTH IS CORRECT**
Cut a 194mm (go a little longer if in doubt, you can adjust later) length of the 32mm tubing.
Put 20mm (or whatever you feel comfortable with, my measurements are based on filling the bottom cap with epoxy) of epoxy in the cap that you made from the Reducer (seal the outside of the "wire hole" with hot glue first).
The Button Contact should be just above the epoxy.
Put some epoxy on the outside of the tube as well (rough up the outside and inside of the tube with sandpaper first).
Push the tube into the Cap and look to see if the Button Contact is square in the bottom of the tube. Once it looks good leave the whole thing to cure.
Epoxy the 32mm Access Plug into the 32mm Coupling (very straight forward, remember to rough up the contact faces and keep the threads clear of epoxy. You'll be needing all the threads to get a good seal on the rubber face in the screw-on cap).
*****Wait 24 hours or whatever your epoxy curing time is*****
Put the Coupler and Screw cap unit onto your 32mm tube and put three batteries in the tube. Hopefully the batteries are about 10mm short of the end of the tube. See how the fit is and adjust the tube length if needed (obviously you can only adjust to make it shorter :o)
When it's good epoxy it all together.
Seeing as the Access Plug has a seal built into the cap...
...it should seal the battery unit up nicely.
Now you have a nice Screw-top battery pack (that looks suspiciously like a pipe-bomb...)