To give you an idea of what I'm talking about, take a look at this playlist I created of various milsim bombs:
All part of the Grey Ops Youtube channel.
If you don't want to make your own device, you might have to settle for a more low-tech equivalent. A Japanese company makes a rather convincing looking fake bomb which retails for 7,718 squiggly things, as seen below:
If the above product is too expensive or superlative for your tastes, you can use a regular alarm clock, a pocket watch, or modify a comically designed clock:
Well, now that I've made a few more watch lists by making a post about "bombs", remember that you can contact me with any harassments/threats/cease and desist orders right here. And I suppose if you have any suggestions, ideas or comments, you can send them there too.