I'd very much agree about building a lift off wooden top for it as a way to protect the plate when it's not in use. Simple Windex window cleaner works well for cleaning them. High accuracy metrology equipment is always extremely delicate due to that built in level of accuracy. Any parts going on the plate should first be checked for burrs, contamination such as chips etc. so your nor scratching the plate surface. Machined parts, height gauges, surface gauges etc should be slid onto the plate from the plates edge for a couple of reasons, it lessens the chance of dropping any part or tool onto the plate, and will tend to help wipe any dust or contamination that might have been missed off the bottom surface. Air born dust is a constant issue with that level of accuracy as well, if my plates been uncovered and exposed to the open air for any real length of time, I clean it again before it's used. And for my surface plate, absolutely nothing goes on it that doesn't need to be there, and for the exact reasons I even have a surface plate. I've seen various Youtube videos showing surface plates littered with tooling that had zero need to be on the plate, that practice can help create damage and extra wear on something that needs to be protected and used for the purposes it was intended for.
With an off shore low cost surface plate, there's no way to be sure that certificate is trustworthy or not. Very very few of the one's buying these plates would have the experience, knowledge and expensive equipment available to properly double check those accuracy claims on that certificate. My Vertex rotary table also came with a fancy certificate of accuracy, except for the few items I could check such as the table flatness and Morse Taper run out didn't match what the hand written test certificate says. So I'm highly suspicious of it's worm and worm wheel accuracy claims as well. Years ago I did buy a fairly cheap pair of 1,2,3 blocks that claimed to be at or under .0002"guaranteed accuracy, years later I actually had the proper equipment to double check that, very surprising they did in fact match what the manufacturer claimed. But and depending on what you bought that plate for, it's actual level of known accuracy may or may not be all that important. If it's only for simple marking out of parts, it will be fine. Checking tooling and parts for how flat they might be or for hand scraping machine tool parts to a high degree of precision and alignment is a different story. Part and hole layouts are fairly inaccurate, multiple times less accurate if you still center punch hole locations. That practice is still good enough for most fabrication type work, but for fully machined parts, it's a technique that should have died out in the late 1800's. With my feed screws I've personally checked, or my dro, I only do part layouts as a visual double check I'm making my cuts on the correct side of a complex part or in the waste material.
