Hey nitro man! I know you said you wanted to make it but you might find this interesting. Check out the link for this K&N Filter for rc cars. It says it will fit .12 .15 & .21 engines. Not sure if you want to fab your own but here is one option. Never used one so cant tell you much about it. There is a thread about a similar intake that someone made and it looks cool. You might want to read that thread and see what everyone has to say about these types of filters.

http://cgi.ebay.com/K-N-RC-Air-Filte...QQcmdZViewItem

Good Luck!