The very obvious question is why don't you buy a solar cover to start? It acts like a greenhouse and traps heat in the water, plus it insulates it at night when the sun passes.
You may also want to look into the coil/pyramid type of solar heaters like a Solar Pro that have a 3x3 footprint per unit. I don't know how well they work but they are compact and several (like 4) would be surprisingly small. Combined with a solar cover you should see significant heat increase during that early part of the open season.
But I would suggest starting with a solar cover and a solar reel to make it easier to cover and uncover. I like the transparent ones, blue or clear best. I've found they last 3 years whether I buy a heavy expensive one, or a less expensive lighter one, that's their life span.
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