I've made 'em in the past with an old spark plug, knocked out all the porcelain and threaded for 3/8 NC. Use a 4-5" piece of 3/8" allthread, available at any hardware store or NAPA.

In reality, I have to agree with Jack. Buy one from Summit and have it shipped to you. Here's a budget unit for $6.95
http://store.summitracing.com/partde...5&autoview=sku
Or you can get a really nice one with a hole drilled in it to let the air out as the piston comes to the top like this for $11.50
http://store.summitracing.com/partde...5&autoview=sku