I've never been good at calculating bends. Otherwise, any 3d modeling software works. You could just use cylinders instead of empty pipes if that doesn't matter.
There is FreeCAD and Fusion 360 available for free on the web. I believe Fusion 360 only limits you on the total number of projects you can have active at once. FreeCAD is free, but I found it to be a little harder to use than Fusion for some things. FreeCAD has good tutorials to get you started if you go that route. Idk if Fusion has tutorials, I already knew how to use Fusion before jumping in. Best of luck!
Sketchup is pretty decent and there should be a few browser version on their website. It's super simple to use as well if you're just going to be drawing a basic structure you'll only need like 3 tools.