It's a little obvious there isnt it?
If it's gonna be that obvious and clear to see,... then at least make it to where it's not easy to spot who did it. As in, for example, the spy's sapper.
All the spy does is come up to the sentry and click and etc and it just fixes itself on the sentry.
Shit I don't know.
The thing is, if it's a fixed position, the innocents will ALWAYS look there before they check the body, unless they do not know about it.
How about, wherever the traitor places the bomb, it goes to that position? Also, make the C4 smaller, like the size of the worldmodel (AKA:When you drop it, it's a tiny little C4 that looks cute and completely non-deadly xP). But with this, you would have to make the blast radius smaller, how about it just kills anyone within identification range?
EDIT: However, there's a problem with the manual placing of the C4, it would perhaps take too long and a innocent will just see them planting it and kill them before finishing it :/