The main thing to keep in mind is that the neck of the Howell throttle body is 5-1/8" (5.125"). This is the same size as many carbs, so you can go from there and customize your filter setup to your desire using whatever size/shape carb style filter you like. The larger you can go on filter size, the greater the filter area, and thus the longer your filter will last without needing to swap in a new one. If you go off road, you might want to avoid the K&N style washable filters. They let a lot of small dust particles through.
I just picked up a round, generic carb air filter right off the shelf from my local parts store. Just been using regular STP brand filters for it. I think mine is about 11" diameter and easily clears the brake booster. I needed a neck spacer to better clear the electrical connectors on the throttle body.
Another option is to use a plenum type setup which would allow you to position the actual air filter in a location other than directly over the throttle body, let's the engine get cooler air if you position it right, and you can better protect it from water. A little more pricey than the carb style filters though.