If you cannot prevent use of the torch in a restricted possibly hazardous place you must provide shielding to flammable/fragile materials. Sheet metal, wet rags etc. serve the purpose. Fiberglass doesn't burn readily but the heat WILL pop the gelcoat.

It's better, if possible, to make up the assembly in such a way that final tie-ins can be made where the hazards don't exist. In your case, PERHAPS a few couplings off the stubs of the mixer would allow that.

If you go the in-situ route, follow up with a fire watch. Too many house fires from carelessness here.

"Sheet metal, wet rags etc. serve the purpose. "

Or a ceramic blanket designed just for the task.

They are about 10 inches by 12 inches with a lead backing to allow you to form them as needed and hold the shape desired.
A torch for soldering pipes will make short work of damaging a fiberglass surround.

Use enough torch to allow faster in and out and less time with the torch near the fiberglass.

I use an air-acetylene rig since I already had acetylene tanks fr an oxy-acetylen welding an cutting torch.

The use of a torch with a hose makes things a lot easier since it works in any position.

That's a tough project to learn soldering on. People who are adept at soldering don't hesitate to tell others how easy it is... especially the quick in and out and tight spaces with fragile surrounding material. It is easy when you have the right tools, the knowledge, AND the experience.

Sounds like you ought to consider a plumber... might be money well spent.

"It is easy when you have the right tools"

Mostly the right tools.

A smallk propone cylinder with a torch head screwed directly onto it is one of the worst things.

Everything works adequately on 1/2 inch pipes, and maybe 3/4 inch pipe with two joints on a fitting.

A 3/4 inch T is very difficult with an inadequate torch.
The flux burns out of the third joint as the other two are completed.

The lead free solder is a little harder to work with (it does not flow as easily as lead based solder) but has gotten better over the past 5-10 years with improved alloys (a very small amount of silver is one of the additions).

If you are very green at seating pipe a plumber might be worth the price to avoid any chance of damaging the surround.

I'm with justalurker and brickeyee on this one. I don't think this is the place to learn.

Having that valve as your first solder project is like trying to write a master's thesis when you haven't had freshman English. Hire a plumber for this project and pay him some extra to go slow and show you exactly what he's doing. It's not a substitute for doing it yourself, but every bit of in person visualization you can get will help you.

For a first time solder project, pick up some pipe and fittings and practice on creating a copper pipe trellis for your garden. It doesn't need to be water tight, and you won't have to worry about a restricted work area. Then move on to more challenging projects like a pipe that will be under pressure and so needs to be absolutely water tight.

Thanks, everyone, for the tips. Little did I know that DH watched a YouTube video, and went ahead with the soldering within an hour of my initial post. He's a pretty careful guy, and skilled with his hands, so I can hope it went well. According to him, the surround looks fine . . . we'll know if it's sealed when he pressure tests tomorrow morning (with air). I'll definitely insist that he get a heat shield of some kind to use if he needs to add more solder if the pressure test shows a leak.

Thanks so much!

If the pressure test shows a leak you cannot add more solder. You have to take the joint apart and do it over again.

"If the pressure test shows a leak you cannot add more solder. You have to take the joint apart and do it over again."

THIS.

Adding solder will not work.

It will not flow and close the leak.