Design
I’ve said it before, but the best thing about making your own gear is that you get to design it just the way you want. In my case I wanted something that was easy to detach from the bike when I got to camp and easy to put back on in the morning. I quickly realised looking at the different takes/solutions out there, that I really didn’t want to have to take of the entire bag very often. Furthermore the hassle of making and waterproofing a full bag didn’t really appeal to me. It seemed to be a lot of work for uncertain returns. If someone out there has tried both the harness type bag and a full bag, I would be very interested to hear the pros/cons of the two in comparison to each other.

Side by side look of the finished bag with and without drybag.

So what I opted for was a harness type setup with three points of contact with the bike. two velcro straps attaching the harness to the seat post and one attaching the bag to each of the two saddle rails. See the close ups below for details on attachment points. The harness bag means I need only loosen one side strap and undo the single buckle on the back in order to remove the waterproof bag from the harness. It also means I can pack the waterproof bag inside my tent, on days when it’s raining or the ground is wet and mucky. And if the waterproof bag should starts leaking it’s really not that big a deal to go out and buy a new one, compared to reapplying seamsealant or patching up a full bag.

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Size and dimensions
I’m not a trained seamstress or designer in any way or shape. So I use a very low tech approach to bag design which starts with a raid on the recycling bins for some big pieces of cardboard. I then lean my bike against a wall and hold a piece of cardboard up behind the space where I want my bag to sit. I mark out an outline of the space between the wheel, the seat post and the saddle. This is the space I’ve got to work with, yours will likely be different from mine, unless you’ve got my exact bike and saddle height. The taller you are the bigger the bag you’ll be able to have, due to the increased clearance between tire and saddle.

Now that I’ve got an outline to work within, I overlay that with a sheet of baking paper and begin working on the design of the bag itself. I had quite a clear idea of how I wanted the bag to look and work by this point. So I was mostly focused on placing the “contact points” correctly. And how far along the “wings” should be placed. In hindsight I would place the “wings” further up towards the top of the bag, I feel like this would provide even more support and structure.

After finishing the design I transfer each of the three different shapes onto another sheet of baking paper and cut them out so that I can transfer them onto cardboard and use as these as guides to draw around. Do remember to flip the cardboard model to get a left and right side, it’s surprisingly easy to forget 😉

I double layered the sidewalls to create more structure, but I’m not sure that it was really necessary. Maybe if I was working with fabric that was less stiff I would’ve really needed to double up.

It’s a bit of a fiddle going around the narrow bit, but it really does need to be quite narrow. Of course it will depend on the width of your saddle and indeed the width of your thighs, but still for most people I suspect it will be quite narrow.

I have used this on more than one occasion already and it’s works a treat. I’m so pleased I’m going to make a slightly larger version to take on longer trips. I was worried when I made this smaller version about how stable it would be and how much weight it would be able to support. And so I limited myself to a quite short and small waterproof bag. I’ve since grown more confident and honestly for a longer trip I could do with the extra space.

Bonus packing advice
It’s quite important to pack the waterproof bag so that it keeps its narrow profile at the end that attaches to your seat post otherwise your thighs will rub against the bag. And while it’s not the end of the world, it’s quite abrasive and I feel sure it might over time ruin a nice pair of bib shorts.
Looking at the first image below, you can see that I’ve not quite got the packing right, it sort of bulges a bit. But what I’ve found is that my cook pot actually has the perfect dimensions for filling out that space just at the front end of bag. It’s got just the right size and width, and of course it keeps that shape.
I’ve also found that using a waterproof bag with a little valve that lets out air as you compress it, really helps get a good tight fit. I was able to get quite a good fit with the blue bag below after a bit of practice. But with a valve bag I was able to get a better and tighter fit and it was a lot easier and faster to do so.

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