If the cabinet is made out of wood and the walls aren't three foot thick solid oak, it shouldn't cause any significant issues. More likely culprit is signal interference. 802.11n is more tolerant to these things.
You could also try changing the channel and see if that helps. If you're on channel 1, move to 6, on 6 move to 11.
Depending on what router you have, you may be able to perform a survey to see how many other access points are operating in your area and what the noise floor is like.
If you really think the cabinet is an issue, you can get an separate antenna (assuming PCI) and mount it outside the enclosure. Something like
should do the job.