Cardboard would be a pretty good solution. It’s resilient but also would have a bit of give to get it into the soft case. You could laminate several layers to get the strength you want in the areas where you need it.

Or…. A friend of mine was moose hunting several states away and the weather turned unexpectedly hot. When he got his moose he needed a really huge cooler to get the meat home in. He got a couple of 4x8 sheets of XPS foam-board (I think he used 2” thick), and a can of Great Stuff insulation foam, cut the foam into top, bottom, and sides, and glued them all together with the spray foam. I think he used duct tape to hold things in place until the foam cured. As I recall he did all this either in the home center parking lot or at his campsite right in the back of his truck. He ended up with a cooler that was sturdy enough to hold meat and ice and big enough that it almost filled his truck bed. That moose was delicious!

Anyway, that might be another option for you. Honesty, if it’s something that'll never get wet the cardboard would probably be a more durable choice for ongoing use. I think the foam would eventually break up in use and storage but it’s an easy, lightweight and protective way to build a big box.

Jack

 

I have one of those bags. Any loose areas on the fabric will flap in the wind and destroy themselves. It held up for maybe half a dozen road trips before the flapping started blowing out seams and the inner rubber coating started flaking off.

Maybe look at teardrop camper designs for inspiration. They're fiberglass or paint on thin plywood on a frame and hold up well to the wind.

 

I had a soft roof top bag similar to the one pictured. (My son now has it.) I wanted to make an internal frame, but could not come up with what I thought was a good solution. Plus, I liked the idea of the soft envelope being able to adapt to odd sizes and shapes. I kept the bag from flapping in the wind by using plenty of bungee cords and/or rope to keep things well lashed down. When I was a kid, a car top carrier came off the car in front of us one day. My dad was able to miss hitting it, but there was clothing and other items everywhere. This made me more than a bit paranoid, so I always kept it well tied down.

You might use wood 1x2's for vertical supports and use the fiberglass tent poles to go from side to side or front to back to hold the top up. But I'm not sure the shape would be maintained at highway speeds.

 

Cardboard would be a pretty good solution. It’s resilient but also would have a bit of give to get it into the soft case. You could laminate several layers to get the strength you want in the areas where you need it.

Okay, thanks. I don't eat mammals (or ducks) and frankly am repulsed by the thought of killing anything, so maybe the freezer option wouldn't be ideal for me. But doubling up on the cardboard is a good idea.

Maybe look at teardrop camper designs for inspiration. They're fiberglass or paint on thin plywood on a frame and hold up well to the wind.

Maybe that is the answer - a thin plywood box. I wish someone made one big enough, but so far I haven't seen one.

 

Okay, thanks. I don't eat mammals (or ducks) and frankly am repulsed by the thought of killing anything, so maybe the freezer option wouldn't be ideal for me. But doubling up on the cardboard is a good idea.



Maybe that is the answer - a thin plywood box. I wish someone made one big enough, but so far I haven't seen one.

A rigid foam box seemed like a DIY solution worth mentioning. My sincere apologies for any unintended repulsion caused by the inclusion of its origin story.

Jack