Am I the only one who does not use anything? I do use heavier fabric for my bags and fusible interfacing, they seem pretty sturdy.

I had a cutting mat that was completely worn out, so I cut it into rectangles and used it for the bottom of the tote bags I made. Also made an extra sleeve, so the mat could be removed to launder the bag.

I ask for the heavy cardboard that the Quilt Shop or Fabric Stores throw out when they get to the end of the bolt. It's free for me & the store doesn't have to take it out to the trash. Some bolts have stronger cardboard than others. I'm sure that it is bendable at a certain weight, but so far none of my Bow Tucks bags sag. I've heard of people using foam board from a craft shop, too.

I run a small business out of my home and I get a lot of fed x boxes. I cut them to fit and make a sleeve to put the cardboard in. I have made at least 100 totes and it holds up great and most of all it is free.

I buy my kitty litter in the large, plastic jugs. When empty I cut out the large sides and use them, sometimes doubling them. I also make a "sleeve" inside, so the plastic can be removed when washed. I've also used Very heavy cardboard, usuually 2 layers, one with lines of cardboard running east and west and the other running north and south. Again, making a sleeve so it can be removed for laundering.

Bag bottoms
 

I use a material in great supply after an election: those annoying political signs
made of corrugated plastic! They cut easily, are very stiff, waterproof, and
can be easily removed from the fabric sleeve I encase them. Then I use the
metal stakes to prop up plants. Reduce, reuse, recycle, and clean up litter.
It is especially satisfying to use the signs of the candidates you didn't support...

I used double plastic canvas also but lately I have better results with plastic placemates. I use my old ones but you can purchase inexpensive ones in dollar store. They are easy to cut and some of them don't even need the sleeve and are easy to wipe clean. I do like the finish look so I put them in sleeves.

I go to my local x-ray clinic and ask for the hard plastic sheets that are used for shipping protection of the x-ray film packages. It can be cut to size and it is free.

I'm going to use that quote on my Website if that's okay with you. The one about the kids ... my readers will love it.

Wandering around Home Depot or Lowes is almost as fun to me as the fabric store! (operative word :ALMOST! lol) I found a blank sign- like you might put out front of your house for sale. It is like a corrigated plastic and easily cut with a utility knife. I used it for the bottom of a new tote I made and I love its sturdy lightweight base. Here are pictures of it being cut. The piece was about $5 and I will get about 5 purses or totes, depending on the size. I did cover mine to match, but I dont' think I needed to. I also cut the 'ribs' of the plastic the short way. I am hoping it is a sturdier cut.