Which seating die are you using? The die body shouldn’t need to be touching the die holder to set seating depth, only the crimp depth. If you’re running out of seating stem travel a quarter inch early, meaning the stem “head” is dead-length shouldering against the die body, then you have something out of spec in the die. If using a Hornady die set - Either your seating bushing is too short, or your stem. Inserting a plug in the die would give you additional depth, but I’d be careful to ensure it properly engages both the bushing and the stem, else you end up with some induced run-out. Honestly, if the stem is too short, I’d be calling the die manufacturer - there really isn’t any excuse for a proper spec to have this trouble, so the die is out of spec, and they’ll most likely be able to quickly and easily assist you to rectify the issue.