The "undercoating" I use (Krown) is more of an oil spray. It doesn't matter if the car is rusty (and my car is). The synthetic (rubber safe) oil is thin and soaks right into the rust then right to the good metal underneath and seals it. Krown doesn't dry or harden it always stays "wet" and creeps and seeps into nooks and crannies continuously and displaces moisture as it does.
Some undercoating is touted as "dripless" which is less messy (doesn't drip on the driveway) but doesn't seep and creep or displace moisture nearly as well. It can in fact, seal moisture into the rusty areas where it just keeps rusting.
A third type of undercoating is something like truck bed liner or rubberized asphalt undercoating. That is the kind of stuff you're probably thinking of that requires sandblasting. That type of stuff works well on a new car and the car needs to be completely dirt, oil and rust free which is almost impossible for our cars at this point and it can actually make rust worse by sealing in moisture rust and salt then hiding the decay under a thick layer of "paint". This kind of "undercoating" does nothing for treating what I consider to be the critical areas you can't see like inside the doors, the pillars, cross members, frame rails, etc.
As far as weave and such goes, you'd do that from inside the car along with a nice runny, drippy, oily spray underneath. Your carpets just pull out of the way, then you just lay them back down when you're done. The fiberglass weave is also available at Walmart and Canadian Tire for about $15 for a 3 or 4 foot square which may be all you'd need. It's super flexible and can follow the ribs and humps and valleys of the floor pan easily to make a complete seal. And metal even mesh would have to be pounded and shaped to fit then holes drilled for screws or rivets. The mesh should still be covered with something solid and permanent because the biggest purpose of the patch is to seal the cabin from any possible exhaust leak from entering.
Check out this Krown video,... You may not have a Krown near you but you want a treatment that is similar... With holes drilled and spray put in.
https://youtu.be/NcgNIGqrsGU