I've made pen kits from 338 Win Mag and 300 Weatherby Mag cartridges using a cigar pen kit. 338 Lapua Mag uses the same diameter bullet as 338 Win Mag and is only a little longer than a 300 Wby Mag. As I recall, however, the 300 WbyMag was a pretty big pen. 338 Lapua Mag would be larger in diameter and longer.