It sounds like you did a fine job of constructing homemade in-tank nurseries for your 90-gallon aquarium. A 120 micron polyester or nylon plankton screen would be perfect for containing Artemia. Suitable screen can be purchased from Miami Aquaculture Inc. (see the following link):
The best adhesive to use for screening of the holes you have drilled in your in-tank nurseries may depend on whether you feel the polyester or the nylon plankton screen is best for your needs. The nylon screen is softer whereas the polyester screen is much more durable and stands up to regular cleaning really well compared to the nylon screen. I should think that silicone aquarium sealant, which is designed for use on glass aquariums, would probably suffice, but you may want to contact Miami Aquaculture to see what type of adhesive they would recommend for securing the screen to your acrylic nurseries to make sure it will work well with the polyester or nylon screen you obtain.
I would include some holdfasts in your in-tank nurseries for the seahorse fry to hitch to at night, but other than that, I can’t see any reason why your homemade in-tank nurseries should not work well. Strips, sections, and cylinders of plastic window screen or the plastic mesh sold in craft stores for needlepoint projects make fine hitching posts for seahorse fry. Short lengths of polypropylene rope (the kind sold at hardware stores and marine outlets for boating purposes) are another good option for hitching posts in the nursery. They come in many different colors, can be cut to any desired length, and are buoyant so if one end is anchored and the other end is unraveled, they will wave gently in the current like natural plants. (Avoid nylon rope, however — it bleeds in saltwater and will leech color and who knows what else into your tank!) If necessary, the holdfasts can be secured to the acrylic with silicone aquarium cement or suction cups designed for use in marine aquaria, or secured to a piece of coral rubble to anchor them in place.
The only aspect of your homemade nurseries I am unsure about is the small pump you included at one end. It would have to be very small to avoid generating too much current for the newborns so hopefully you can adjust the flow rate of the pump to a minimum that produces the desired result without buffeting the babies about in the process.
Best of luck with your in-tank nurseries, ageber! Here’s hoping you soon have a brood of babies to occupy them. Please keep us posted on how everything turns out and any modifications you make to the nurseries as you go along.