Hi Ralph,
Mounting 2 vertical 20x60 extrusions onto 20x20 horizontals with free-moving gantry plates is absolutely doable. To help you choose the right gantry carriages, we’d need a bit more info—like where you want the carriages positioned and how they’ll be used—so we can recommend the best option. Please send us a rough sketch of your project at help@ooznest.co.uk
As for locking them in place, you can use sliding extrusion nuts in the slot with M5 screws and aluminum spacers to build a simple stop. While we don’t currently stock hand-operated knobs, you might find “M5 knobs” with threaded studs from local or online hardware suppliers, which would replace the M5 screws for easy manual adjustment.
I hope this helps and looking forward to hearing from you!