weak MDF box, strengthening ideas?

hello people, yesterday i built a large mdf box to suit 2 15" fusion powerplant speakers both are recieving around 400wrms each

the box is 110L give or take 2L

It sounds bassy as hell, but very very distorted, hopped out of my car to find that the box is flexing through each bass hit.

Now the box has a centre barrier so it is 2 seperate chambers, and i have used silicon to properly seal each chamber off from the other. the mdf is 18mm thick all around. I have also used metal brackets to bolt the box together, the box is pretty strong..obvisouly not strong enough though.

now i built this box to fit exactly into the boot behind my seat and above parcel shelf, so everything is spot on, which unfortunately does not leave much room to brace it from the outside (which i believe needs to be done)

What i was thinking of doing was covering the whole box in fibreglass and the parts where there is not enough room for anything bigger than what the box already is, just using resin

will this strengthen the box if i only use 1 layer (which is all i can really afford due to lack of workable space around the box)

what other options do i have to stop the box from flexing?

thanks in advance

Take the subs out the box and try to brace it as much as possible.

ive strengthened it up and the box no longer flexes but the distortion remains.

i have the subwoofers sitting straight up and they are sitting below my parcel shelf, theres a clearance of about an inch and a half, is this good or bad? btw its metal

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