Hi Chris, yes I thought of that and several years ago a German member used to make valve guides like that. The problem here is if the hole for the lower guide has been damaged and enlarged, It would mean making the new gadgets with an oversized guide parts and then machining them for each application once the damage had been corrected. It would also be a problem to machine an internal recess to take an oil seal. It would be interesting, if there are enough examples, to see if there is a correlation between fitted oil seals and this problem.
Instead my gadget is intended to allow a repair with, no access to machining tools, for anyone who has had this problem. Provided that there is enough metal left to fit a new oversized lower guide with a flange then this system works. I am not trying to build up sales for this as you know that I do this for no profit but it might be a good idea for anyone rebuilding an undamaged cylinder head to install these gadgets anyway to prevent a problem in the future.