1/4 barrel sanke for primary?

I'm brewing my 4th and 5th batches this weekend (both AG) and I'm sick of donating a gallon of beer to the brew brew gods. I have three 5 gal carboys and on the last 2 batches I lost close to a gallon each out of them despite using a generous dose of fermcap in the boil AND in the primary.

My kegging setup from craigslist came with a 1/4 barrel 7.75 gallon keg that I'd like to use for one of the beers and I may be able to scrape the cash together for a 6 or 6.5 gallon carboy. I also have 2 empty 1/2 barrel I could use for the other beer. I've seen the kick ass setup Darrin? has but I don't have the time or the cash for that. If I use a carboy cap how do I plug the other hole, the one the airlock doesn't occupy? OG on both batches will be around 1.060ish

Thanks!

I ferment in sankeys and a #10.5 stopper works perfectly.

wort monger does it and he does it under presser with a type of releaf valve