I think you will find the majority of members here will agree that the stock airbox is the most efficient with an aftermarket panel filter, but they are hard to come by.
You can make youself a heat shield using some aluminium sheet from B&Q, I did that myself and found it did help but TBH without an extra intake on the bonnet it's always going to get hot under there. Best bet is to ask around for a stock box or get a local engineering firm to build you a new unit unless your handy enough to do it yourself
Not sure about the N/A's but in all honesty there was hardly any difference in performance on my TT when I took my stock airbox out and put a cone filter on but that's just my experience.
Good luck anyway pal