WARNING! fire or boiling water can be dangerous. kids ask an adult before using either!
i was asked to explain my tequnique for quick and dirty parts fitment,so i'll do my best! i actually had some heatshrink in my tool bag from doing electrical work years ago. the proper way to apply it is with a heat gun. also you can use a hair dryer, hot water ,or my favorite fire! i started doing this method slowly,akwardly, and with some reservations. but soon i found an aplication that really worked well, saved time , and involved fire!
it is a solution to a simple problem. when peg A is too small for hole B. used to i would mix up some 2 part sculpting epoxy and partially fill the hole, then jam the peg in to shape the hole, removing it slowly praying the epoxy stayed put.
now i clip off an appropriate length of heat shrink tubing, apply it over the peg/ball, heat it to shape, and see how it fits. if still loose you can repeat the process. i'm not sure how many times, but probably infinite or until you reach a size where tubing is no longer made, which you shouldn't in this hobby. i have some that is about 2" diameter that shrinks to 1.25" that i've tried using with varied success.
here i will be using the cheapest set i could find. it's from harbor freight tools in the electrical isle. it has 3 sizes that are perfect for our hobby. i will try and post pictures of three different pieces i have mocked up just for this tutorial. two heads and a forearm.
what you need to get started.

head holes

good fit

-cont. bellow. I went this route because I had it almost done and lost it with a careless X-ing

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The more you run over a dead cat,
the flatter it gets.
-Anonymous