If you want a bit more heft/weight to it, then the stainless washer option is good. I'd be using 2 part epoxy glue (Araldite) to fix permanently anything inside the base. Just make sure you whatever use really well, you don't want it coming loose and giving you a rattle for a brush.
Filling it with Araldite is a pricey option, but it will drill out in the future if you ever wanted to. The cork sounds good and light weight.
Be careful with nylon/plastic washers, especially if they are unbranded (found) they may in fact be made of HDPE, and glue does not stick to that.
As for the knot, Silicone for temporary/DIY knot swaps etc. Silicone is easier to release than 2 part epoxy glue (araldite). Use the 2 part epoxy to fix the knot permanently if that's what you want. You can still release the glue for knot change, but it takes longer exposure to hot steam.
Basically, do your measurements, know where you want the floor of the knot socket to come up to, set that with your chosen material, let it set/dry/harden, whichever is applicable, then install your knot on top of it. I wouldn't recommend a one step installation, too easy to go wrong.