Originally Posted by
speakerdave
Consider starting with the JBL 2328 2" throat to handle the transition from round to rectangular cross section and building a wooden adapter to tie together the two rectangles of the throat exit and the horn entrance. You would need the dimensions of the horn entrance and the 2328 exit, the relative size and the aspect ratio of both; the latter are available in these forums. Beyond this basic idea I cannot offer any more help. You would need to work out an exponential expansion, I think. You could possibly use the termination of your Altec throat as a model for the angles they used at the horn entrance. A little fudging may be unavoidable; try to make smooth transitions. The 2328's are neither rare nor common, but usually available somewhere on the used market.
You should also be aware that the TAD 4001 when used on its stock horn requires compensation in the high pass of the crossover to deal with an upper treble peakiness.