One way of providing attachments for chains is to make a channel in the piece. This is most often done by placing something between the glass pieces to maintain an opening for the chain to slide through. The materials can range from toothpicks, coated wire, rolled ceramic fibre paper and many other things which will stand up to the heat for the required time.
One of the requirements is to prevent needle points and drawbacks of the glass. There are a number of ways to do this. Some of them are noted here.
One method is to make pattern bars with a channel through the whole width of the bar. Then you need to cut the bar into strips (leaving the channel material in place), do any edge work required, and fire polish.
When creating a single piece from cut glass parts, you need to ensure the upper piece of glass extends beyond the lower piece by at least 3mm to allow the glass to bend over the channel and touch the lower piece. A little more than 3mm will allow the upper glass to curve over the bottom piece and create a rounded top with no evidence of the joining of the two pieces of glass.
Another method is to use two pieces of 2mm glass with full pieces above and below. The narrowest piece of glass will be about 3-4mm and placed at the top of the pendant. The largest piece will be long enough to give a 2mm gap between the two pieces. This is kept open by inserting two pieces of 1mm fibre paper into the gap. Then cap with the top piece of glass. All the glass can be of 2mm thickness, as the three layers will give the desired 6mm thickness.
Finally, a tack fuse firing can help to avoid the needling that can occur at the channel, as the glass is so much thinner than the 6mm required for a full fuse. This means that you can do the work in stages. First fire the elements to the desired state, then combine them for a tack fuse when creating the channel.
If you use a clear middle glass, you can create a depth by having a design on both the bottom and top layers of glass.