I’ve had issues with bulging in the lower compartment of my frame bag so I built this contraption. Hard walls prevent bulging and straps prevent max. expansion.

If items in your frame bag cause bulges that touch your calves or it expands too wide so that it interferes with your cranks, I can recommend building something like this.

The first version that I used for a few years was just made out of of PVC hard foam board with hand-cut slots for the straps. The straps are just gaffer tape. This new version is 3D in PETG-CF and a little stiffer and more temperature resistant than the PVC version despite being 1mm thinner.

• I flipped my frame bag inside-out and measured the dimensions.
• I also measured how wide I want the compartment to be
• Then I cut a PVC board (or in this version designed a 3D model) to the exact shape for the inner panel.
• The outer panel has to have the same shape but with a section of the top cut off to create clearance for the zipper and access into the compartment.
• Then I cut slots for the straps. For the handmade version I just drilled the corners of the slots and cut the rest with a blade.
• Then I attached horizontal velcro strips with double-sided adhesive and staples (painted black with sharpie). The velcro is the stuff used for cable ties.
• For the straps I used 50mm gaffer tape cut in half to make 25mm strips.
• I used a spacer for the inner space when installing the straps so I get the perfect and uniform length all around the perimeter.

And now the compartment is rigid. No more bulging or expanding too wide.