I asked this question on Twitter about how I could make some simple 3d drawings of stacked cubes and received some interesting suggestions and also later looked around and found some other tools that were good, many free!
This Isometric Drawing Tool produces exactly the sort of simple 3-d shapes I was looking for and is totally free. I like the flexibility it gives for inspecting the final product and letting you view a clean version of the final diagram with no axes etc. There are lots of other free tools here too.
The one that was closed to what I was looking for was the free tool here https://www.mathspad.co.uk/i2/plansElevations.php . There were actually loads of other tools here as well that I haven’t explored yet. Looks to be a great value subscription with many tools available for free.
There is a great free tool at https://mathigon.org/ that allows you to create all sorts of 2d and 3d diagrams but also so many other things too.
Geogebra’s 3-d calculator allows you to draw 3d diagrams on a set of axes that could include stacked cubes https://www.geogebra.org/ .
There is a free manipulatives tool that can create some nice diagrams here https://mathsbot.com/manipulatives/blocks