Who made or used a 30 round 8mm Bren? Smaller magazine carry boxes for 20 round 8mm mags - for Chinese / Inglis 8mm guns. Larger 30 round mag carry boxes for .303 (several UK/Commonwealth versions) Redesigned larger UK versions for Commonwealth .308 30 round magazines. Presume that there was a bulk mag container for the 8x56r metal 30 round mags which would have to be about the same dimensions as the standard 30 round .303 - the mags are very similar in size... but I've only ever seen the loose magazines and not a carry box for them.
Short answer for the original poster... No, 8mm and .303 Bren mag boxes were different sizes - a smaller 20 round straight sided box vs. a 30 round curved banana mag.