There are D-I-Y plans to build a lift boom on the N*T*C* site, under HOW-TO's/Implements. Curious though, what's a tow bar got to do with a boom? You can do anything you want, it is your tractor after all, but I'd say unless you already have the material and welder lying around, why not just go to a boneyard or auction and get a used one or any dealer will sell new ones. I have bought these for $25 or less at auctions.
Tim *PloughNman* Daley(MI)