My husband has some left over formica, kitchen counter topping pieces. He cuts them for me, a little smaller than my bottom and rounds my corners. I then make a little 'pillowcase' for it, out of my lining fabric. It sits inside when you have completed bag and can remove for washing.... Don't know if your husbands a DIY like mine or not....could also use something called luan, a very thin plywood but comes in 4feet by 8 feet sheets, was about $10--$12 a sheet and not heavy as you just need a small piece.

Plastic canvas is quite good.

Here in Australia you can buy replacement plastic bag bottoms pk 5 at the supermarket for their reusable bags.
I just cut them to size and make sure the corners are rounded.

some of our home improvement stores cut plexiglass for free.

bought some plastic desk pads and cut them to size, rounded cornes and pillowed like Toogie

I used plastic canvas last time, but if making a few it could be costly.

Round off the edges a bit and no poking through. Hugs. Nan W

If you do plastic round the corners that way they will not punch through the fabric.

Another thought: cover your card board with old shower plastic liner using the upper part that has not been use much round those corners also.

I use the bases from my local shopping bags once the bags have worn out!!! ❤️ Chris

I used some thin plastic cutting boards. Worked fine. (bed bath beyond) but check the dollar stores.Also some covers from a spiral notebook.(Cheap 89 cents each)Stella

I use flexible cutting boards, cut to size with the corners rounded. A local home improvement store runs them on sale quite often, 4 for 99 cents. I just picked up 2 pkgs yesterday.

I use a rigid plastic table set: I cut it with scissors after having marked it with an indelible marker (pen) and I insert it between the bottom and its lining.