I agree with everyone that if you have a definite work area size you need then design is critical.
When i built my cnc i had in mind a 500 x 500mm cut area. I was on a budget as most people and was building using both new and used components.
I ended up with 400 x 320 cut area by the time i had finished.
My machine dimensions are 1100 x 750 and its a moving table type.
Please keep this in mind as its something i learned at the end of my build but wish i knew earlier, especially because you want to cut aluminum.
Most diy builds you won't see any way covers of any sort.
You want protection for your rails and screws.
So design them in at the beginning.
I added mine last and lost about 100mm travel.
You can get them custom made in china for a good price just be aware that you need to add roughly 50-100mm each side of your axis rails so they will fit or you will lose travel.