I've used a schumacher batt. tender on my boxster and motorcycle. I got it at autozone for like $24. I'm very satisfied with the quality of it! Here's a link to what they look like.
http://www.technooutlet.com/egle9345.html?mr:trackingCode=B3631332-53C5-DF11-907B-002219318F67&mr:referralID=NA