This is an experiment (so not quite perfect!) but interesting I think. The inner clear 16mm nominal acrylic helix (tube) is bought for the purpose from a Scottish supplier. This is inserted in a 16mm hole drilled in from the base end before turning. The outer wooden helix is made (after turning to shape) using a router and a jig called a 'router-crafter' that I bought three decades ago (before I had a lathe). The wood is an old chopping board supplied by my sister - supposedly teak but looks like mahogany.
Yes the helices are deliberately of opposite senses (the acrylic is as supplied but the 'crafter' can do either) - not like DNA I know but I thought this might look better. There is a slot under the base for an LED tea light shining up the tube but I am not quite sure about this! Pics on request.
Comments welcome!
For anyone interested the wooden tube is 25mm dia (16mm hole) so <5mm wall thickness, so I made a sacrificial softwood 16mm rod insert to stabilise it when using the crafter. This might not be necessary but it worked.