'what is a good size / length of template - I have done 7cm x 3.5mm and 18cm x 3.5mm'
Well, here goes. In for a dime, in for a dollar. Rightly or wrongly, this is what I see:
This is short and narrow. 7cm x 3.5mm is 2 3/4 inches x 1/8th inch. This length is good for tight spaces or for a smaller scale, I think. To me, it would take forever to rivet a 1/24th or 1/18th if one has to relocate the template after every 2 3/4 inches.
1/8 inch seems narrow to me. Vertically: At 1/8 inch wide, and there being 3 perforations in vertical alignment per intersection, each intersection is a weakness. It is a kink waiting to happen.
Horizontally: At 1.13; Why .13? Why not .10? or even 1.00? Both divide into 1.00 evenly. Does the .03/.13mm difference really matter? Real life tolerances found on manufacturers' drawings, drawings used to make items, not construct them (construction drawings), have tolerances greater than .03/.13mm, allowing for expansion and contraction. Two rows of perforations 1/2mm apart is a weakness and a bend waiting to happen.
The product needs to have elasticity, to be able to bend, and with all of those perforations, I think stainless steel would be about the only material that wouldn't accidentally kink. Material thickness would help to minimize accidental bending.
18cm x 3.5: I think is a good length for 1/32nd and has use in 1/24th and 1/18th scales. It's 7 inches long. For 1/18th, where the horizontal stabilizer is probably 7 inches long, and a wingspan and fuselage each are close to 24 inches or some such thing, I think it is workable.
I think what will determine this products' success or failure is not its' idea, but its' material and thickness.
Just my take. Other's mileage may vary.