Second auth layer for HTTP control panel with Raspberry Pi

Hi guys, I have hdmi modulator with GUI web-based management panel and I need to organize secure and robust access from remote network, I don't want to use straight NAT because you can "brute force" the native auth panel. I need some kind of layer between remote user and NMS via gateway, which will manage logging and auth.I was thinking about 1 channel router(maybe raspberry pi based)... What to choose and from where to start? Thank you!