I used to do it much more than today. My first was to follow an article about building a new nose for the P-36 to make it into an XP-37, I carved Balsa, sealed it with Model Airplane Dope as my sanding sealer, and "bake molded" a clear canopy from some plastic draped over a carved wooden buck. I did progress to entire aircraft, but, when I got to the wheels back then, I needed something more solid than balsa wood,,,,,so I sawed "wheel blanks" off of an aluminum rod, I sanded and filed on those discs until I had a tire and wheel with a depression in the middle.
Just about the time that I got really serious about all of this,,,,,I turned 18 and joined up,,,,,then I had enough money to just buy all the kits and conversions I wanted. So, for a good long while, my scratch skills set dormant, with some of the quickness being lost.
Now, today, I want more versions and weapons than what are being offered in plastic and resin, so I buy the aftermarket stuff,,,,,,,,and scratch build a lot of the more unique weapons that I want. Being able to start with an aluminum tube or plastic tube or rod, and all of the other shapes we can get now that we couldn't back in 1968 has helped a lot. (an old memory for some of you,,,,early gray Plasti-Strut couldn't be glued with the Model Shop glues we had back then, only Plasti-Weld worked on it,,,,,then the newer "hot" cements came around and I think the plastic formula was changed)
It is a big boring list for people that don't need the stuff, but, almost every US aircraft carried a Practice Bomb Rack* since 1960 or so,,,,,,and no model company has ever released one for sale. There are about 20 "no brainer" things like that, all of them have to be scratchbuilt. (*Not the same as the Practice Bomb Carrier Pods)
I bet TB and Don see "something" in ordinary shapes as they walk around in the non modeling part of their lives,,,,,,I still do,,,,,,even drinking straws look like incredibly tall gun tub shields if you stand them on end, lol.
Rex