IMHO- juice ain't worth the squeeze. There is expense to do it right and I would assume you are speaking of putting boxes in a SUP or surfboard that wasn't designed for foiling? What you end up with is a board that isn't very good for foiling. If you are learning- having the right board will make life easier. It made more sense when the sport was brand new and there were limited options on the used market. There are now a fair amount of used foil boards available- you'd be much better off going that route. Have fun!