IF, you lower the top onto 2x4s so that the cables are no longer supporting it, there is certainly no strain on the cables. You'd really need to cut the 2x4s so that they have to be kinda forced into the space between outriggers and the top with the top in the full up position. This will keep the bulb seal against the upper lip of the lower half.
If the 2x4s are too short, then supporting the top with them will leave a gap between the bulb seal and the lip, which will allow insects or other unwanted critters a point of entry.
I my own opinion, I don't really see that supporting the top with the cables for a long period puts undue strain on them. I think each cable is rated for about 400# continuous load, but the breaking strength is on the order of 2000# per cable. I would not hesitate to leave mine up on the cables, but supporting the top with 2x4s is not going to cause any problems either, if they are the correct length.
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