To add to this thread, I took another user's suggestion and cut out the cardboard from water bottle packaging. I wrapped it in an old tshirt and will use for a few days to see if I like it. It was a great suggestion and fits nicely in my bag.
Same as everyone else, I cut the proper size from a white sturdy piece of cardboar and taped the sides to prevent scratching. I'm going to wrap it in some fabric but maybe later. For now I have an extra scarf which I layer over it to make my bag look lovely :)
I used to use a magazine as a shaper but that added too much weight to my bag.
Hey guys, if you want better material other than cardboard although maybe not as sturdy, try those vinyl placemats from the dollar store, or plastic chopping mats (search eBay, dollar store).
If you go to those arts and crafts store or a hardware store, they might have those plasticy boards that are thicker. I don't know what the material is but it feels/looks like plastic.
Also, BALSA WOOD! Typical arts and craft wood that kids use for school projects etc. Or those cardboard sheets architects use to make house models are great and have awesome finish to them.
I don't know if this would work, but polycarbonate sheets! Those things people put on roofs for insulation I think? This may not be as accessible for most people but for those who have men who hoard things in the garage, they might have access to these hahaha (my uncle had some, no idea why).
Once you find a good material, its a matter of just cutting and fitting. I've been looking for alternatives to buying base shapers online because some of them are rip offs, and you don't really know the quality of it before you buy. If you make them yourself then I guess you can use what you prefer and get a feel of it before you make it.
I have made base shapers out of carboard for my speedy. And rounded the edges of and then taped the edges for that exta security. I have had no problems with it and it makes my speedy have a polish look to it. The corners of the cardboard are rounded and taped so it doesnt damage tge corners of my speedy. :)
I remember my SA telling me if I want a base shaper to use the LV box you get when you buy your bag and cut the shape out and use it as a base shaper. I tried it for a while on my Speedy but I found I love slouchy sag look much better.