The only kit that comes to mind is an AK kit screw-build which would include bolting the rails in place. Whether you fold a flat or carve out an 80% with your Dremel tool it still fits the criteria.
A round tube semi-auto like from a Sten MK-II kit will require some careful silver braze and rivits as well as store-bought semi-auto pieces. A MK-III kit using a MK-II mag housing might save some effort on the front end built as a pistol.
If there were a super simple/easy kit as you wish the gun nazi feds would be all over it big time reducing it to crumbs.