I planted all my garlic cloves this past fall and winter (2015) in raised beds. They are all doing fantastic. It's easier for planting, weeding, harvesting, and keeping nutrients contained to where you need it. Also easier to amend.

Peter - Are you planning to use this new raised bed for garlic every year? I would probably go with a mix of 50% sand, 25% garden soil, and 25% compost if it was going to be just for garlic but that mix would also work well for carrots, beets, or other root veggies. With 6" spacing you could get 49 garlic in a small 4x4 bed.

I actually have a big bag of sand sitting in my shed. 50% huh, that's not too much?

How can I isolate it from the soil below, should I build a platform? The idea is I don't want it to catch anything from my soil. If I put plastic under it then it won't drain...

Yes I was going to use it as a specific garlic bed... but I could probably slip a Fall crop in between harvest and planting. Maybe even carrots.

I think sand works very well for garlic. My brother's property in Wisconsin is in an old floodplain and his soil is almost 100% sand that he amends with compost and composted manure and he grows some great garlic and doesn't even have to dig when he harvests. I guess if you put plastic under the bed you could drill drainage holes on the sides of the frame at the bottom. If you are going to put a bottom on it I would go at least 12-15" deep.

I grow in 3' in diameter rings of 2X4 fencing which are lined with landscaping fabric. Use light weight hardware cloth for the bottoms. The contents of the beds is completely potting soil with added nutrients. I get 3 dozen garlic give or take from each bed. I also grow in 18" cans and get 10 to 12 heads from each of those.

I would not worry about isolating from the dirt. My beds are at least 14" deep and most roots don't go that far into the soil anyway. Have been doing this for 18 years in the same place.

I have what looks just like white rot for the 2nd year in a row, so I definetely want to isolate the bed.