The basic idea is to make a bag shape out of both fabrics then slot one inside the other and join them around the opening. This means there are no rough seams on the inside. It also means you don't have to sew through more than two layers of fabric which is important if your fabric is thick and your sewing machine less than industrial.
The design was inspired by this site. We liked the design and there were heaps of helpful pics through some of the links on that page.
We actually made this in two steps. We completed the bag in one sitting without too much thought of the strap system. We then shopped around for strap hardware and designed the strap system afterwards. It worked out ok but if you are going to do something different with the straps think hard about how it will work before sewing anything.
Step 1: Gather Materials
fabric for outer - we used wool felt ~1yd
fabric for inner - we used black vinyl ~1yd
velcro ~1/2 yd is enough
bias tape - for making a neat edge - I got wide double fold from JoAnn
some nylon webbing
some reflective tape (optional)
We got all of the above from a local discount fabric store here
Materials for strap system:
2in nylon webbing
1in nylon webbing
2in cam belt clip
1in side release clip
2 2in plastic swivel hooks
We got these from REI and a local outdoor fabric store.
Step 2: Make Pattern
Once your pattern and dimensions are sorted out you'll want a full size pattern. We used baking parchment to do ours. Take special care to get your lines straight and angles square. Mark on the pattern which edges need seam allowances (marked in photos). Cut out.
Step 3: Cut Inner and Outer Fabric
It's really helpful later if you transfer the edges of the pattern onto the fabric on edges that need seem allowances. I got a neat little tool and special transfer paper from my Mum. It looks pretty retro but I'm sure you can still buy them. You basically place the transfer paper under the fabric then run along the edge with the little roller and the help of a ruler and it transfers a line to the fabric.
Cut around the pattern remembering to leave about 1in seam allowance on the necessary edges - I forgot two :(
Repeat steps for outer fabric.
Step 4: Add Velcro and Pocket
Put the inner fabric on top of the outer fabric and fold both into the bag shape. Lay out the velcro where you want it and pin to one layer of fabric. We put the fluffy velcro on the inner fabric and the pricky side on the outer fabric. Sew the velcro in place.
We put one pocket on the front inside of the bag. We just took a rectangle of the wool felt and hemmed the top edge. We then sewed the other three edges onto the lining fabric (we didn't bother hemming these edges but you could). We also ran a few vertical lines of stitching to demark a few compartments.
We also added a strip of reflective tape to the outer flap (no pics here but you'll see it later).
Add any other decoration you want. The idea of this step is to attach everything you need to the two layers before we start making things 3d.
Step 5: Sew Bottom and Side Seams and Add Webbing Loops
Now pin the long sides together including that little triangles that attach to the flap. Now is a good time to put in any hardware you need to attach the straps. We just put webbing loops into the seam at the top and bottom of each side. Sew the side seams - go over the webbing parts a few times.
Repeat with inner fabric - again with the good sides together.
Trim the seam allowances to 1/4 inch. Then turn each right side out.
Step 6: Attach Outer to Inner
Once done you'll have a messenger bag with some rough edges and no straps.
Step 7: Finish Opening Edge With Bias Tape
Step 8: Add Straps
Hopefully you can figure out our strap system from the picture. Using plastic swivel hooks made things easy. We just hooked them onto the webbing loops sewn in earlier. It also means the strap can be completely removed or flipped around.
Sew one end of strap to triglide. Feed the loose end through swivel hook and back through triglide. Then sew this end onto the cam belt.
Sew one end to other swivel hook. Feed loose end through cam belt.
Stability strap at shoulder strap:
Sew one end to shoulder strap. Sew the other to a side release clip.
Stability strap at bag:
Sew one end to webbing loop on bottom corner of bag. Pass other through the other piece of the side release clip.