I am looking for a handled clamp or a wrench like tool to hold the shower valve (here, a Symmons single handle Temptrol) still while I unscrew the front cap (here a Symmons part T-12A)., to replace the cartridge.

With the escutcheon plate removed, the entire valve body and about an inch of each of the 4 pipes (hot, cold, shower, spout) attached to the valve are exposed from the front.

There is no structure behind the fiber glass shower stall within 8 inches of the shower knob on any side to have fastened the valve body to. Only the pipes hold the valve in position and just trying to unscrew the cap with just one wrench threatens to tie the pipework up in knots.

 

Build a wood anchor for the valve body or have a helper hold the valve body as you loosen the nut.

 

What is on the back side of the valve?

The faucet body should be securely mounted for just this reason. I would cut open the back side of the wall so you can install a cross brace between studs to mount the faucet body. Your super lucky if a closet backs up to the shower so you could install an access panel, and it doesn't have to be pretty. If it's a hallway or other room you can cover the hole with a nice cover panel or repair the sheetrock.

 

The back side is not available. This is a condominium and the back side would be the next apartment unit. Incidentally that unit has its bathroom and its shower valve directly behind, also with no structure supporting it.

 

I hate to say but more than once I've had to go back to my shop and make the tool I needed.

 

Are the valves in the same cavity? Can you see the adjacent valve by looking behind yours? Is the condo new or a conversion in a rehab building?

I would expect a new condo to have fire separation between units that would block you from seeing the adjacent valve.

If the wall cavity is small enough (and restricted to your unit only) you could possibly figure a way to squirt some expanding foam in and around your valve/pipes/etc. that might hold the valve solidly enough to prevent it moving. Do not fill the cavity completely with foam in one shot as it could expand too much and distort or break the wall surface.