Customized version of http://www.thingiverse.com/thing:1654016

Created with Customizer! http://www.thingiverse.com/apps/customizer/run?thing_id=1654016

You can buy these clear tubes for protecting size T12 fluorescent lamps at Home Depot for $3, they are sold near the light fixtures, not the bulbs. They make good storage for some things, but the caps that come with it have holes in them (see the black one in the picture). I cut one tube to 20" and printed the red caps. Thanks to Magonegro.

I printed with the rings down and the cap up. It bridged over the top fine with no support.

Using the following options:

Fillet = 6
InnerMAXDiameter = 41.5
CAPHeight = 10
RingHeight = 3
Facets = 100
CAPOuterDiameter = 45
InnerMINDiameter = 39
RingsRatio = 1
InnerHoleHeight = 16
Rings = 3
InnerHoleDiameter = 35