Improvised tire tube - GIF

[IAMSatisfied]

The "hard" part of the job is going to be the rolling resistance a wheel like that is going to have being so soft.

Several years ago I bought some solid "tubes" for my wife's bike, which essentially turned it into a real workout to go anywhere (yeah, I purchased the correct size for her wheels). I ended up going back to pneumatic tubes with Armor-dillo thorn-proof liners. Pneumatic wheels are are difficult to equal or surpass from a practical engineering standpoint.