Once you have the strap the "slingshot" can be made quite easily. from spare workshop materials Wood, plastic, metal tubing or metal rod, sheet metal, etc. can all be used to fashion an effective slingshot. Matter of fact even the strap is pretty easy to make... surgical tubiny and a small piece of scrap leather. .
Oh and as far as the law goes remember, it is not a "slingshot". It's an "antenna installation tool". ;)