Can I make a DIY alternative to the Nikon SG-3IR flash cover for wireless flash triggering?

Actually, some people have just used exposed/developed film as a filter, but in these digital days, that stuff's harder to come by. Infrared filter gels are probably as or more expensive than the device you're looking at.

But generally speaking, most people never need this device for two reasons.

First, light from the pop-up flash rarely registers in off-camera flash photography, because the optical master/commander signal is done at a low power level. Because of the inverse square law, used at normal subject distances, this light tends not to register much if at all in the final image, compared to the main flash burst. And if you're shooting at macro distances, then you're more likely to be using a ring-light setup or some other on-camera combination of flash.

Secondly, optical systems have weaknesses. They require line of sight and low ambient light levels to be really reliable. They work great in studio conditions; not so great out of doors in bright sunlight without bounce surfaces around. For these reasons, most off-camera flash shooters use radio triggers. And you can choose to have a "headless" (i.e., a unit with no flash lamp) unit as your on-camera transmitter. This won't emit any light at all and doesn't use the pop-up flash, so eliminates the need to make the master/commander signal non-visible.