Thanks Jamie. I planned to pretty much copy Drumroll, haha!

I was going to do exactly the same. The shaft traveling the length of the robot and securing into HDPE/other plastic side bulkheads (I actually have the shaft and bearings right in front of me, lol). My problem is... well maybe I should have phrased it differently... I meant, I have a metal tube, basically a toilet roll out of some kind of steel. The problem is mounting bearings into the sides of it strongly (cheaply!). I planned to cut a circle of HDPE with a diameter the same as the inner diameter of the tube, then put a bolt all the way through it an the tube, securing them together.

I can't do that though because it would clash with the shaft. So I wondered how I'd make a strong but lightweight and affordable join of the two pieces. I'd simply press fit the bearings into the HDPE circle, but attaching that circle to the end of the tube is my problem.

It sounds like it should be easy, I know, but I have no access to precision tools, so cutting a perfect circle out of HDPE is already a challenge! Ah well, all part of the fun I guess.