in similar situations I have used portions of felt pen caps as inner caps. You sacrifice the felt pen cap, cut a frustum of the hollow conical cylinder the wider diameter of which corresponds to the portion seated on the section-nib rim, and the length of which should keep being sloughed off to create a secure closed fit. (Measurements help, but trial and error often is needed.) Then you close off the top part of the frustum first with tape, and then layers of silicone glue that are dried to create an umbrella. With the final coat of either silicone glue or shellac, or simply a press fit if there is enough snugness, one stuffs the inner cap thus fashioned.