Lately I've been taking a different approach when I make a flat horn pattern versus the paper exercise (I grew weary of making the paper to conform to the horn). I boiled up a small horn I was going to flatten and threw it in the bench vise (woodworking) at nite and next morning used the 'flattend' horn as a tracing pattern on a piece of wood. Then I make some length and width adjustments of the pattern to be sure it will fit inside. After making my kerf cuts and rounding over all the edges I will then make a couple of hardwood wedges for use.
Be warned, you may have to boil, clamp, re-boil and clamp again to get the horn to 'obey'. Most times this will happen the thinner you want the horn to be, e.g., half inch or thinner vs. 3/4 inch. I'm not talking the thichness of the form, the total width of the base plug plus the horn.
Gary