I just did them on mine.
I ordered the parts from Aircraft Spruce, a 2x2 sheet of alclad 2024T3 (I went with 0.050 thick), and a package of 50 K1000-08 nutplates and 50 screws (8-32 8R8 screws). The kit comes with a bunch of nutplates and screws already, so you may be able to get by using those first and reordering later.
I didn't do the mod until about 2 months into it, after I already had things together, then apart, then together again, just before putting on the front side skins. I'd recommend waiting just a little after the kit shows up in case you have anything else you want to order from Spruce, you can bundle it into the same shipment. If you just order them from Vans and have them ship it in the crate that may make sense as well.