Thanks, everyone.  The bag worked out pretty great using some hdpe I found at a fabric store.  I like the idea of using cheap sled material, and will go that route if/when I make more.

 As people described above, I left the fourth edge unsewed until I right-sided the bag and then just sewed that edge by hand.  Worked pretty well.  I've just taken it around the neighborhood but seems very stable. 

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I have used HDPE from Tap Plastic for some of my items.  Not sure if it will work for you: https://www.tapplastics.com/product/plastics/cut_to_size_plastic/hdpe_m_m_opaque/258

Thanks!  Will try. 

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I'm planning on sewing a toptube bag, and I'm just wondering what you all recommend for a stiffer material to give the bag some structure.  I went to my local Michael's to look for something, but struck out.  Any recommendations? 

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Hi,

I'm looking to build my first frame bag, and was wondering what the best way to handle the velcro straps.

Are there any advantages to sewing material onto the back of the velcro before attaching to the bag? I see some bags with and some without and not sure what the advantage of added material is.

Also, what would be a good length length for the velcro straps on a steel road frame and 3" wide bag?

Thanks!

I don't sew fabric onto my Velcro.  What I do is just to tack a piece of webbing along the bag so that it acts like a daisy chain.  That way you can move the Velcro around to see how it fits best.  This is also good if you are competing for frame space with a top tube bag or something.   

As far as the length of the Velcro, it depends on the bike and tubing.  I use double sided Velcro tape, and I just measure and cut it when I'm first putting the bag on. 

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