Horn-Loading a Delta 10

I want to build some new boxes for my 4way PA (mid-high), consisting of Delta 10 + 1.4" (altec 299). I already built the Dual 15 which will cover the 80-250hz area. The box I need to build must have the outside dimensions of 36" x 21 1/4" x 30" and 13 3/8" rear width. The horns I have are the Altec MR564 which is a 13x13 horn. I could use some other horn, but for now that's what i have. And that leaves me with roughly 21 inches of total height for the Mid Horn. The HF horn is a 60x40 horn, so the mid horn should be something like that. Is there anything that can be done for the Delta 10, in such space left? I already built the outside box, so all I have left to do is make the mid horn and stick it in there. Can anyone help me, or does anyone have any ideas??
BTW, I have no experience whatsoever using hornresp or designing horns. I have tried using it, but seems kinda complicated. Thanks a lot for your help

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