This is my DF F4D 'Skyray', with a 'trimmer' attached with the blue masking tape. I'm told they were used to hold nose up attitude when in the pattern so as not to use the elevons. Also, I read they trimmed the plane, which had a tendency to 'tuck under', when going super-sonic. Think they will have any effect at our speeds?

I would say that it would be very critical to find a material or arrangement that has "just" the right amount of flex.

Frankly for a model I'd say you're barking up the wrong tree. For the most part our models tend to fly with more rearward CG's then the full sized stuff and so the need for elevon trim is minimized. I think you'll find that you can set up your model to fly just fine without these difficult to setup tabs by just using a semi rearward but still stable CG location coupled with a minimal elevon up trim. Otherwise setting these spring tabs up will likely be a bit of a nightmare.

PS: THat's one FINE looking project. All the best with the test flying.