I'm guessing you built out of 1/2" plywood... pick up enough 3/4" half-round hardwood trim equal to the height of your J15 times 4, plus another 12", and a 3/4" hole saw. Cut 8 1/2" thick pieces of the trim. cut 4 pieces the lenght of the external height of the cab.
Build your array. Use the array to mock the placement on the outside of the J15, and make a simple drilling jig (this is important). Use the drilling jig and the hole saw to make 2 holes through the "top". Carefully mark and drill through the "bottom". Insert the half-round trim, using the array as an angle guide. PL the half-round into the drilled hole, and immediately add in the 1/2" wide cut-offs. Allow the PL to cure, sand and DuraTex the new array mounts, then attach your array.
I know money often seals the deal, but seriously, quality is an investment, not an expense... Grant Bunter
Accept the fact that airtight and well-braced are more important than pretty on the inside. Bill Fitzmaurice