I'm make 2 laminated labels. One to put on the main 150A breaker coming into the house and a 2nd to put on the main panel. I'll put these in place and turn off the respective breakers before plugging the generator in. These will stay with the generator when not in use and will be my manual "lock out, tag out" when in use. It's manual, but should prevent any issues for my personal use.
Seriously, don't do that. If it isn't mechanically interlocked with the main or switched by a two-source throwover, then at some point, it will
cause inconvenience, injury, or death. You may have a safe approach to using it but you have enabled a fundamentally unsafe configuration, which someone else may attempt to use when you're not there to supervise. Most utilities now require their line workers to use hot-work precautions or apply grounds before engaging in any maintenance, but during post-storm recovery when lines are crossing or dropped, all bets are off as to what is connected to where.
Envision yourself in court hearing your manslaughter conviction while a dead neighbor or lineman's family watches from the gallery, and then facing a million-dollar civil suit out-of-pocket after your insurance company invokes the gross negligence clause. Or, if your generator is accidentally energizing a line and the utility can't readily identify the source, your section of the circuit may have restoration delayed until they can unwind it. Now your neighbors are waiting longer to get their service back, and you may get fined by the utility for the added trouble.
These are worst-case scenarios, but there are several possibilities that have to happen exactly once and suddenly your savings on a $500 transfer switch and code inspection are turned upside down by at least 1-2 orders of magnitude.