You don't need a filter with an airbox. If you are running open carbs & are happy with that, then an airbox with no filter is no different. Just remember to clean it out before a run.
Surface discharge from NGK £25GBP in UK, there are other makes that may be cheaper.
http://www.sparkplugs.co.uk/ngk-raci...-plug-r0045j-9
Are then any airtfields near you ? find a drag or LSR crowd & approach them to use it if its a low use military field etc.