You can build a "half" adapter cable
Cordage is special cable specifically made for flexible cords for AC mains power. Anything with a "cord and plug" uses it. Toaster, desktop PC, iron, plug-in washing machine, refrigerator, you name it.
Cordage uses the same amps as wall wiring (actually a bit higher sometimes) except when it is specified, the ground wire is counted. /3 means black white green. /4 adds red.
It is legit to make your own cord.
You would need 10/3 cable at the least, which is expensive, but using 10/4 might be wise so you don't have to buy the cord again if you ever upgrade generators.
On one end, you need a TT30 plug. On the other end, an L14-50 socket.
Ground to ground.
Neutral to neutral.
Hot to one of the hots. Pick one.
You must run this with all 240V breakers switched off, including the ones you do not realize are 240V breakers :) Otherwise really weird stuff will happen. Wouldn't start a fire, but might damage motor appliances.
You will find half your 120V circuits work and half do not. If that is satisfactory to you, that is fine. If you'd rather the other half be powered, then change your adapter cable to connect to the other hot instead.
Why not split hot to power both sides? Problem. Some houses have shared-neutral or multi-wire branch circuits (MWBC). This only works because the hots are on opposite phases. If they were on the same phase, neutral would be overloaded. If you can review your panel and prove your house is MWBC-free, then you should open up the inlet and join the hots at the inlet wiring... not in the adapter cable, because it might be used on somebody else's house!