There are some pretty decent cheap bike lights available on eBay, but they have the disadvantage that the battery packs are not waterproof, they're just shrink wrapped and tossed into a cheap nylon bag.This thing as designed holds a standard 4 cell 18650 pack, which is the most common one. The SCAD can be modified to hold other sized packs.It includes a strain relief for the wire which has a kink in it to crimp the wire so that a tug on the wire should not rip the wire loose from the battery pack inside.Also includes a pair or rails that will keep the pack centered on a tube of the bike.Instructions:Print both halves. To make a printed object waterproof, use a cotton swab or brush to apply acetone to the outer surface. Note that this may cause white spots on the printed object. I did this three times, after the third the spots were pretty much gone, but the object was waterproof.Insert your battery pack with the lead exiting the case along the strain relief end. Test fit to make sure everything works. You may wish to insert some padding if the pack is loose, so that it doesn't rattle around. Apply silicone sealant along the seam, be generous especially at the wire exit point, and use self tapping screws to secure the lid. Press the strain relief tightly together to crimp the wire. Wipe off the excess sealant and wait for it to cure.You may wish to cement on a bit of old inner tube or other soft material over the rail side where it will touch the frame of the bike to keep from marring the bike and to keep it from rattling.Thread a velcro wrap through the holes on the side.Enjoy!