[Elmer-Dallas Texas]

Some grab bars have end caps. You can cut the bar to any length you want. Easy!

 

[matheny00]

Remove the fiberglass shell. Imo installing a fiberglass shell was the first mistake. Fiberglass shells waste room and adds to claustrophobic feel.
Then put up back boards where you want grab bars. Then cover the walls with solid material that is Corian type or manufactured marble.
There. Problem solved! Thank you.

You are right they waste room but are a cheap option when money is tight.

 

[matheny00]

Did you bother to read the op? It is a contoured fg stall that sits 3" off the studs and drywall behind it. While I would agree that it is not as secure as a bar fastened directly to studs, that is not always an option.

I did read the OP's post and there are 2 solutions that IMHO You can open the back wall up and put blocking in or buy The Solid Mount on Amazon and do the modifications from the manufacturer's website for spaces up to 3".

 

[Elmer-Dallas Texas]

You are right they waste room but are a cheap option when money is tight.

Fiberglass shells are lousy imo and OP question points out why they shouldn't be used. Space is valuable. And the shower is a very important space that people use every day.
I grew up poor or at least we pinched money like we were poor. Our shower walls were made of laminate. Laminate does not have the problems of fiberglass shells.
Grab bars are important for safety and should be attached securely to studs imo.

 

[iamrfixit]

Fiberglass or acrylic units are economical, quick to install and very easy to take care of. They rarely develop hidden leaks that rot out your house. I tore out more than a few of the old tubs or showers that had laminate, hardboard or plastic surrounds. Always had leaks and hidden damage. Nasty looking caulking and sealants all over trying to keep them sealed up.

I ripped one of those out of our house right after we moved in almost 30 years ago. Installed a fiberglass tub/shower that's large and spacious, used every day and it still looks brand new. No leaks, ugly caulking, mold or mildew. It's easy enough to install blocking and grab bars during install of the unit, if you have the forethought to do it. It's not the fault of the product if the installer failed to do it.

 

[Elmer-Dallas Texas]

Fiberglass or acrylic units are economical, quick to install and very easy to take care of. They rarely develop hidden leaks that rot out your house. I tore out more than a few of the old tubs or showers that had laminate, hardboard or plastic surrounds. Always had leaks and hidden damage. Nasty looking caulking and sealants all over trying to keep them sealed up.

I ripped one of those out of our house right after we moved in almost 30 years ago. Installed a fiberglass tub/shower that's large and spacious, used every day and it still looks brand new. No leaks, ugly caulking, mold or mildew. It's easy enough to install blocking and grab bars during install of the unit, if you have the forethought to do it. It's not the fault of the product if the installer failed to do it.

Fiberglass enclosures virtually never utilize all the space available when used as retrofit. They are claustrophobic imo.
My showers built of sheet goods have had NONE of the problems you state. As you said "It's not the fault of the product if the installer failed"
I cringe every time I see a fiberglass shower stall because of its looks and functionality. The danger due to improperly installed grab bars is another reason. Fiberglass does leak and causes rot when it cracks due to stress caused by stepping into the shower.