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I am more of a "junkyard", but the part I plan to make seems difficult to make from sheet steel, because of its curves quite complex for a non-sheet metal worker ... 'a shell of about 25x12 cm, the exact shape of which has not yet been precisely defined: the prototype would serve precisely to adjust it according to the tests that would then be possible to carry out (I am convinced by the interest of the 'empiricism). It would require a fairly good mechanical resistance, even if the duration is not necessarily a crucial criterion given the vocation of this part to give the right information for the final realization.
I had thought of synthetic resin, like that which is used for the realization of orthoses or splints, but I do not have someone competent on hand ... Maybe epoxy resin + fiberglass? It stinks, but it's accessible and can be applied on a plaster mold or other ...
I had also thought of glulam (it's more in my strings), but the piece has curvatures in several planes which seem to me to present difficulties which are hardly compatible with this process ...
Do you have any other suggestions?

Completely off the cuff, because not necessarily having understood the purpose and the specifications of the "object": a block of solid wood that you would machine?

For my part, the statement is too vague for me to have any idea and offer an element of a valid answer for sure.
Afterwards, as a jeweler, I made pieces only by hammering ductile metals: gold, silver, copper. Copper is relatively easy to hammer to obtain concave and convex (associated) surfaces because it anneals easily, regains its "ductility" and off you go for a round. We can hammer on more or less wet clay, for example, since we can reuse this material indefinitely, in jewelry, we did that on "pads" (masses) of lead which we gave a "neutral" shape by the after. Once the prototype is made, it is easy to make a mold with various materials: plaster, resin, for ex.
You can also make your proto in "solid" wax (not beeswax), make an impression, a mold ... In fact, I do not understand your project well.

The object that I am trying to achieve roughly resembles a barber shampoo tray / is also much smaller and it is the concave part that is important. The idea is to get something not too thick and with a side that conforms to the desired shape, the back does not have this requirement ...
Sicetaitsimple, regarding the block of wood, yes, it would not be a bad idea, because it is a material that I think I can work quite easily ... I only miss a gouge, but it is a detail that can work it out (or with a chainsaw! ).
Guy, copper does not inspire me too much, because apart from plumbing, I have never used it; moreover, I would need a certain rigidity ...
What do you mean by "solid" wax? It reminds me of stucco: I had already made a mess (I don't remember at all what ) with and it was quite interesting as a material, because it can easily be grafted onto a metal or wooden frame, moreover it is similar to what I said above about resin orthotics, but in a more old ... I think it is or was not badly used for the sets of theater ...

What about 3D printing? In plastic the resistance is not terrible but it is very suitable for parts with complex shapes ...

There are fablabs that have resin 3D printers ... see metal ...

The disadvantage obviously is to have to model your part.

Here it seems to me that it is 5 € per hour for a pla or abs printer ... see what's in your corner ...

And in concrete ??? or in modeling plaster ...

3D printing indeed supposes a modeling for which this proto should be used, so it is not good, Christophe.
Yes, Macro, I already thought about it (see above) about modeling plaster. I think that a flat wooden base on which would come to superimpose the desired shape (which is not very pronounced) could be suitable quite well. In short, a few avenues to explore ... The hardest part now is for me to reflect and validate (if possible!) The functionality of the system where this part must play its role ...
Thanks in any case for all your ideas and for the help I get from them!

Ahmed wrote: I only miss a gouge, but it's a detail that can be arranged (or with a chainsaw! ).

and / or small burs on a drill.

Ahmed wrote:What do you mean by "solid" wax?

This is the sculpting wax used by jewelers to make their lost wax model.

In the recovery materials, you have the wet newspaper (soggy in the fleet for hours). (Not newspapers with glossy / photo paper)
It ends up making sheets as coated with mud / glue it takes the shape well and hardens as it dries.