The F-91W has 18mm lugs, but it uses a friction-held (non-spring) bar, which limits your options. Further limiting it is that it doesn't quite look right without the flare out and taper. Some people don't mind this and swap it with a 18mm NATO strap and just wear it like that indefinitely.
I solved this problem by going on Aliexpress and ordering replacement metal band that fits the F-91w, A-158, and similar models. It costed $4.98. Been using it ever since and it works great. Being metal, now it's the least fragile part of the watch. You can also just upgrade to the ~$20 A-158, or one of the other metal band Casios, and get that out of the box. Timex also makes the T80 with a metal band as well.
I slightly prefer the light weight of the plastic band, but if you're okay with a metal band and your arm isn't too hairy (these low end metal bands like these can be painful if you are), then that's an option. The only tool you'll need to replace this is a spring bar tool that has the straight poker end or something similar.
I solved this problem by going on Aliexpress and ordering replacement metal band that fits the F-91w, A-158, and similar models. It costed $4.98. Been using it ever since and it works great. Being metal, now it's the least fragile part of the watch. You can also just upgrade to the ~$20 A-158, or one of the other metal band Casios, and get that out of the box. Timex also makes the T80 with a metal band as well.
I slightly prefer the light weight of the plastic band, but if you're okay with a metal band and your arm isn't too hairy (these low end metal bands like these can be painful if you are), then that's an option. The only tool you'll need to replace this is a spring bar tool that has the straight poker end or something similar.