The i-Box is extremely cool, I have made several small items with it and the joints are flawless. The fingers on the i-Box will flex under pressure however so, even with all the great features it will still suffer under a heavy hand. I think some of your challenge is a large 48" panel of 3/4" ply having a fair amount of mass. You could be mis-aligning things without realizing it as the mass of the board hides the force being applied(?).
"A hen is only an egg's way of making another egg".
– Samuel Butler
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Box joint jigs can be out a bit and still be perfectly serviceable. If you are actually trying to do 48" wide joints, then it can't be out that much!
In your case, over 24", you're out 1/4". Over 12", you're out 1/8". That's a LOT! Even over 6", you're out 1/16", and in oak, that's a NO GO.
The variable here is the indexing pin. The blade isn't cutting any narrower or wider on on either piece, so your index pin spacing is off, or, your index pin itself is too narrow, allowing slop in positioning.
You don't need to purchase someone else's jig to get a perfect fit @ 48". Get your pin the right width, and set it the proper distance from the blade, and start making trial cuts on scrap. Go for 6" first, then 12", then 24", etc. You really don't even need two pieces of wood to test this for getting it close. Cut one end of a board, then use your tape to see if the cut is adhering to the expected markings.
Todd
(I have successfully cut 36" joints with a jig I made out of scrap in about 15 minutes)
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