Well we know MDF is the wood of choice.
We know chipboard is the crappy 2nd cousin of MDF when it comes to enclosures 'cos its just not as dense.
Well all know to seal a box air tight.

I like to seal the enclosures edges with Liquid Nails then fibreglass it up to get a perfect seal and stiffen 'er up.
IMO, it doesnt matter if you use chipboard or MDF. If you're fibreglassing the enclosure it'll seal all the leaks, and stiffen it greatly. You just need to be generous with the 'glass.

But, how much is enough, and how much is too much and a waste of money ?
Anyone else got some idea's to chuck in.. baffled edges and slots are kinda no-brainers .

I think you'll find that in SPL circles,
MDF necessarily the "Choice" material..

maybe for costs purposes..
But alot of people prefer hard wood. Atleast for the top panel of the box.

As any serious SPL competitor will know, it takes 100's of hours to tune and get a set up just .

And screwing/unscrewing into MDF over time, isn't good, and the MDF strips. So using Hard wood as it's definate advantages.

But

dont use large sheets of 16mm, it flexes

but IMO, 18mm is enough for a single sub ~3cf. dual sub (~5-6cf) i would brace the middle... f/glass is just rice and a waste of money l/nails seals it up good enough... you have a big port for the air to escape, air is not going to try and force it's way out of a 0.0005mm air leak when you have a 100sq in port is it?

for spl boxes 25mm is fine, maybe 32mm for large surface areas or use bracing. as for fiberglassing, why not, it cost's stuff all and ensures a perfect seal and adds strength.

Here is my theory based on 0 hours of testing..

The stronger the enclosure is, the larger you should make it and vice versa..