You could just get 2 pieces of PVC pipe (I use that stuff for everything), each 10 ft long and run a rope thru them. Then tie one end of each to the back corners of you big boat. The other end will go to the padeye on the front of the TI, or safer yet wrapped around the front AKA brace (one on each side).
This will help the TI track better, the V shape formed by the PVC pipe helps keep the TI centered, and also prevents it from climbing up on you when you slow down (picture in your mind a wood chipper). It's only about ten bucks worth of PVC, and should help. I would first try about 1 inch OD (whatever that size is), if the class 40 stuff doesn't seem strong enough you can go to the thicker class 80 PVC, but I don't think it will be necessary.
If you go to the front AKA brace (the strongest point on the boat) you might end up needing 15 or 20 feet, this is kind of dependent on how far your motor sticks out from the back of the big boat. It would be a shame to chop up the TI with the prop ( like going into a wood chipper).
I did something like this but the TI was the pull boat, and we pulled a pair of kayaks lashed together all loaded up with scuba tanks, coolers, etc out to dive sites (we called it a party barge). I had the PVC pipe tied to the eyelet on the back of each AMA, then to the bow of each kayak in kind of a V shape. They actually tracked pretty well when we were sailing off shore (maybe 5-10 miles out to Sand Key in Key West) 2 foot rough chop. It wouldn't have worked without lashing two kayaks side to side, kayaks are not stable enough and all your gear tips over on a single kayak even with small waves. the PVC I used was only about 3/4" dia OD (don't know what size that is), and wished it had been just a little stronger.
There were 4 of us scuba diving, the other couple had their own inflatable kayaks and they just hung onto the handles of the TI (not tied), which seemed to work out best. Were going to do it again next week down in Key West for mini lobster season (but not on the coral reef, that's illegal).
Hope this helps you