Too big enclosure for sub woofer?

Last year had a lot of bad luck making my bike sound system. I made some boxes, fried some electronics, etc. The boxes were too big for easy handling, so I'll make another attempt, since spring is coming.

I noticed the speaker (driver) did not like higher volumes; very large excursions in the lower Hz range , probably hitting the end of its moving range.

It is a closed enclosure, no port. The RMS output of the amp is probably many times lower than the RMS rating of the speaker (driver); logitech 2.1 computer amp with consumer grade sub speaker (6").

Same problem occurs with my home 10" 60 W RMS driver in a closed ca. 70 l - ca.15 Gallon barrel, with a modest 25W RMS amp: I cannot even turn it halfway up:  it is mounted horizontally, and shoots peas 40 cm (16") up!

Would it be more helpful to make the box / barrel less square? Like halve the size by putting a diagonal separator in the box? Or is geometry irrelevant and does only volume matter?

Perhaps there even might be a formula????