My Joann's has plastic canvas. I used a dbl. layer covered with fabric in the bottom of my first Bow Tucks & wish I'd bought more for my second one.
I sure admire those of you who have piles of Bow Tucks now. I had as much trouble with my second one as I did my first. I think it's because I thought it would go faster therefore, I made some stupid mistakes.

I sew a removeable fabric pocket and use cardboard, I try to re-use everything I can :) I have never made a bowtucks but have made lots of others,it works great for me.those I have gifted the bags to don't seem to mind .If I were selling them I may have to come up with something else but for now cardboard works!

I just use heavy card board with a fabric cover.

Vicky

Vera Bradley bags, which are pricey, use a heavy dense cardboard (like what is in on the back of a note pad, but maybe 1/8" thick) inside a fabric sleeve.

I went boating with friends a couple of years ago, and my bag (which was a gift) got wet. In trying to dry it, I found the cardboard, which did not smell too good w/ the lake water & wasn't washable, either...

I also use heavy cardboard in the bottoms. Free and easily replaced.

Our JoAnns has the plastic canvas.

I use foam core board, 1 sheet makes a lot of bag bottoms. I make a sleeve that can be removed and washed as needed.

I use masonite board....DH cuts it to size for me.
it runs me about .50/bag
I sew a pillowcase for the board that is removable so that the whole Bow Tucks bag can be washed.

I've actually been scrounging around for anything economical. I've got some heavy cardboard....I work with a bunch of guys, and one of them brought me a political sign...I think i can use that once it's cut to size. Any heavy cardboard I find goes home with me...I figure if I'm making a 'sleeve' for them, it can be just about anything sturdy enough! :)

I use the foam core also. I am lazy though, when I make my sleeve to put foam core in I just make it longer and tuck the end into the sleeve. I haven't had a problem with that method. It just saves a little time when finishing. It can easily be removed to launder the bag.