Well, we're basically dealing with a 1 WL of 300 Hz/4 = 75 Hz = ~58" dia. wave, so ~29" from the horn centerline down will drape over the mids and part of the subs, so ideally need to square them up and use closed cell foam insulation or similar to seal the joints, then a 36" w x 36" rounded up and seal the horn/baffle joint.

This might be sufficient, but it's now going to reinforce some part of the sub's, mid's output, so it ideally needs to be well braced, recommending 'boxing' the horn in [open back] to the depth of the mids cab and placing concrete bricks or similar on the bottom plate to mass load it. Better still, Velcro one or two to the horn.

Reinforce the back perimeter, toss in enough damping to mute any hollow sound and the sum total will for sure load to < 75 Hz, which would need to be measured, but a basic pipe end correction would be ~29" x 0.613 = ~17.8"+ ~181 = ~198.8" = ~13543/198.8 = ~68 Hz, so pretty confident it will load at least this low as the sides of the mid/tweet cab will contribute some also even though at 90 deg.