Just saw this and wanted to let you know you can cut up an old cutting mat and use that as a stabilizer.

Thanks for the link....

Take apart a store bag and use it for a pattern

I use the kitchen shears (I keep a pair in my sewing room) to cut up old cutting mats...no saw needed. They are great for bag inserts. Also, vinyl flooring is great too & can be cut with the kitchen shears. Very inexpensive. I cover with fabric & they are removable & can be handwashed...I make almost all my purses washable. I have even prewashed (on handwash cycle in machine) faux leathers & just air dry...makes those purses washable & air dryable too.

I have used this one too and really like it!

I use plexi glass for the bottom, I make a open ended envelope for the bottom and slide it in. That way I can take it out and wash the bag in the machine.
Never thought of vinyl, have some of that in the barn...uhm...lol

You can also use a heavier stabilizer in the sides for support, this will help even more than the bottoms being heavier. Craftsy.com has some free tutes on making bags and like others have mentioned google it.