That's the back-draft preventer. It does, however, restrict air-flow slightly in the forward direction and might act as a surface which collects condensation.
Shop-keepers and yourself are going to want reasonable profit margins, plus you'd have various expenses such as materials, storage, energy, distribution, product liability insurance. What would it cost you to actually make the tool?
That's not a reliable indication of what might be behind the plasterboard. If you're going to be fixing things to walls, a metal/pipe/wire/stud detector would be a worthwhile investment.
Methinks a lot would depend on access. How tall is the garage? Is there space around it for steps/ladders? Will work platforms be used? How many work-people and how fit are they?
How about some polypropylene tape/webbing?
This would be fairly low friction to allow for pipe expansion/contraction and its softening point is 130C, so ok for CH pipes.
Would it be so bad if the shed base were above the level of the existing slabs? Might even be an advantage to prevent water running across the shed floor.
Perhaps because you didn't give it enough time. Water has to flow through a tiny pinhole (which has a plastic pin in it) to reach the far side of the diaphragm and build up pressure once the small pilot valve is closed by the float arm. Only when that pressure is sufficient will the diaphragm...
I checked appliance consumptions the first day I had a smart meter. They're unlikely to change. I've not felt the urge to re-check frequently. My IHD stays unplugged most of the time.