add the following to your engineer.cfg
//Build & destroy
alias "+sentry" "destroy 3"
alias "+dispenser" "destroy 0"
alias "+entry" "destroy 1"
alias "+exit" "destroy 2"
alias "-sentry" "build 3"
alias "-dispenser" "build 0"
alias "-entry" "build 1"
alias "-exit" "build 2"
bind "F1" "+sentry"
bind "F2" "+dispenser"
bind "F3" "+entry"
bind "F4" "+exit"
It doesnt just drop a sentry, but it does auto destroy your old one and whips out the toolbox so you can drop it when you like.
Also works for dispensers, and teles..
f1 sentry
f2 dis
f3 ent
f4 exit
good luck mate