Most of you may already be aware of this.
A company in Bellingham, WA makes a tube sealer for caulking tubes and other adhesive containers called “The Little Red Cap” You can google it and get more info if you wish.
I got a pack of 16 from my local ACE Hardware for a bit more than 3 bucks!
They have about half the pressure of a pen bladder or surgical tubing tank, and are ideal for small engines that do not have fine thread needle valves. I am running one on a Cub for Early Nostalgia and it works just fine
Unlike the penny balloons I was using, Nitro does not affect them so they will last a long time, much like the neoprene pen bladders from Pendemonium.com.
I used a large straight Foremost fitting from the local rc store with a length of large silicone fuel line on the longer nipple. The “Cap” is secured by a #8 Dt rubber band doubled over about 4 times.
These seem to inflate very spherically as well. No area of “dead” uninflated bladder.