Your math is off
piR2 = 3.1415(5)srq = 78.5 sq inches or .55 sq ft.
piR2 = 3.14.5(9)sqr = 254.46 sq inches or 1.77 sq ft.
that means that to scale the enclosure to go from a 10" to a 18" you need to increase the sizes 3.2 times.
in other words your 36X36" box just became 115X115". I hope you have a semi to put this in.
[quote author=Matthew link=1160835651/60#60 date=1164573473]Did you scale everything? Hehe! ???
10" WO = 36" x 36" x 13.5"
Upscale:
18 / 10 = 1.8
1.8 - 1 = 0.8
0.8 * 100 = 80%
36" + 80% = 64.8"
13.5" + 80% = 24.3"
Oh and:
.75" MDF + 80% = 1.35" = heavy*
It seems to me that if you don't increase the material thickness than the opening in the throat will be messed up. Yes, the center or tip of the panel in the throat will be X inches from the corner of the box, but it's width will not be to scale and will change something. Also, the bracing in the rear chamber won't take up a properly scaled volume and will change things yet again.
64.8" x 64.8" x 24.3" with 1.35"
*BONZAI
Well, that's how I would try it anyway.

This box would be helluva tough, but not as tough as he, Mr. T! [/quote]