Create my own shell?

[Abel Talaverón Estevez <abel_(_dot_)_talaveron_(_at_)_openwired_(_dot_)_net>]

Hi all,

I need to create a particular but simple shell for a firewall running OpenBSD 
3.6. The idea is create a user whose shell is a very limited one. This shell 
or command line interpreter (CLI) must have permissions only in the home 
directory.

How could I do this? Any ideas? Editing the source code of sh?, for example. 
Make my own cli?
-- 
Abel Talaversn Estevez
Ingeniero Superior de Telecomunicaciones
Analista de Proyectos
OpenWired, S.L.
C/ Caballero, 87 - 08029 - Barcelona (Spain)
Tel (+34) 93/410 75 70 - Fax (+34) 93/419 45 91