Hi!

Rule of thumb: make the exit hole for airflow twice the size of the entrance. You may have to cut a large hole in the bottom of the cowling. Break the engine in very well before you use it enclosed. I fly without the cowling for ten flights before installing it. Last, but not least, use a fuel with castor oil in it. It will tolerate heat and lean runs better than synthetic. Castor actually increases its lubricity as its temp goes up. The opposite occurs with synthetics. (reference http://aldrich-models.rcplanet.com/castor.htm)

I only use straight castor and though its messy and causes varnish, in 30 years I've never damaged an engine or had to use after run oil and never replaced a bearing.

Good Luck,
Max