I'm not Jay, but here are my comments:
My understanding is that glass or glass matte is basically fiberglass resin and fiberglass cloths/mats which are just applied in layers (a bit like the link i posted earlier to a different DIY thread)
"Fiberglass" is the combination of a type of cloth/mat and resin. Essentially, you "layup" the fiberglass by mixing a batch of resin, then impregnating the cloth/mat with the resin and letting it dry. In the DIY you posted, the guy laid the cloth down first, then added the resin. I prefer to precut my cloth/mat into small pieces, then add resin, then place. I also prefer to use knytex for large areas.
By back, you mean the fiberglass shape thats just been made (i.e the preliminary shape behind the mdf baffle?) Whats the fleece for? and where do i strech it to? This fleece step was missed out on that DIY thread so i can't picture it (by the way i was planning to have an mdf base as well as the baffle)
Yes - the back is typically constructed first, then trimmed to the correct shape. Then you build the baffle. The baffle can be just a ring for the subwoofer or it can be a larger panel like in your example. The baffle is then mounted to the back piece. A gap between the two typically exists. To bridge that gap, you stretch mold cloth (fleece or another material) between the back and the baffle and glue/staple it in place. Then you impregnate that with resin and let it harden. Now you have a firm surface to add fiberglass to and complete the enclosure.
Im assuming that this is just applying fiberglass resin with fiberglass cloths/mats in layers from the inside
This can be done from the inside or the outside depending on how you plan the project. If you stretch the mold fabric correctly, you should have a nice finished shape, so if you apply fiberglass from the inside of the box, you will (theoretically) have to spend less time sanding the outside of the box prior to covering.
Understood that Can i use any glassfibre filler or is kitty hair, like this: http://www.evercoat.com/productDetail.aspx?pID=38 an absolute must?
It's an easier way to seal. You can actually just use a mixture of bondo, resin, and cabosil or chopped strands to make it work. I like to use just resin and cabosil to make a material of milk-shake like consistency and pour that into the box.
Id like to get something pre mixed, as i dont trust myself mixing various chemicals
Everything that you use for this project will require some kind of mixing of chemicals. Resin, plastic body filler, etc. are 2-part products. They all include a hardener (either liquid or paste) that you mix together. The hardener does just that - hardens it within a specific working time. Just follow the instructions on the side of the can and do not mix in large batches - the products will kick faster than you think and if you mix too much, you will be left with a lot of wasted materials.
So what im thinking is to initially measure slightly more than optimum, so i can hopefully get close to the optimum after strengthening
I would definitely measure first, then build. No sense in building if you don't have enough room.
As I'm sure you can gather, there is more than one way to do this kind of a project and everyone does it a little differently. That's not to say one way is wrong, just different.