A good source for all you budding HDPE (HIgh Density PolyEthylene) nav tower constructors is Bread Cutting Boards, yep thats right the ones you can buy in the supermarket for chopping meat and veges or cutting bread on. Come in a range of colours and thickness and easy to work with. They will usually have a 'HDPE' symbol somewhere on them. It is a lot easier than trying to find and source through an industrial plastics supplier if you are not that way inclined.
If using drills make sure they are sharp but also just grind or stone a small flat on the cutting edge axially not on the end. This takes away the 'rake' of the cutting edge and stops it dragging the drill through and into your hand or straight through the seat of your bike that you should not have been drilling over anyway. Trust me it is a good mod to make as it eliminates some nasty drill grabs.
Polyethylene is inherently difficult to glue. Some success can be had with standard araldite but it is not Rally safe. Bolting and mechanical fixing is preffered even if you do glue.
Lets go, here's to the next generation of nav towers