Since you are not using aluminum plates I would recommend that you use .250 or .375 thick acrylic which should be more than strong enough to hold a laser and the aluminum rails, but if you make the foot print too large you will need something a bit stronger to hold up the gantry.
Ya, I decided to use 6mm (1/4") acrylic for now, if I ever add a small spindle, I will upgrade them to aluminum.
Decided to use HPB's modified stl files from his build. He modified them to use the 20X40 extrusion I had. Here is a link to the STL files. https://www.openbuilds.com/attachments/motor_mounts_for_2040_extrusions-zip.20329/ Will post pictures once I have switched to thes 3D printed plates.
Went back to 1/4" acrylic plates, not happy with the 3d printed ones. Need to find some time to finish this project, just the wiring left to do.`
I have all the mechanicals done, and am in the process of wiring it up. Should have some pics of the finished machine in a couple of weeks. Will be posting all the files and a BOM as well.