Here are a couple of ideas.
First one is using the forward hatch hold down cleats. The bolt they use is threaded into a a plastic nut. It has two undocumented (even dealers don't know this) holes in it that you can use to thread line or bunje cord through. The below picture shows the interior nut with a bunje cord I have inserted and secured with a plastic tie for illustrative purposes. You have to look close in the exact center to see the thread hole the bolt goes in. It's easy to see. Just remove the left one to examine. You can also remove the right one to thread something through, but must then fiddle with the hatch.
The next picture you can just see my piece of PVC to house my FF transducer. It's secured mechanically with a hose clamp (which you can just see) to the sailing mast holder. That way (I hope) the silicone seal won't take any movement force/pressure and will remain watertight when I add water (foregoing any effort to use epoxy).
The battery box idea I offer, but don't really recommend
. I did this before I found the poly glue from 3M (see other thread). I took very short screws and used them to hold down the eyelets. I used double overed inner-tube as washers to prevent screw penetration through the hull. I thought I was successful as I could barely detect with my fingertips the screw tip. Alas two of the others then penetrated albeit less than ~1/64. I clipped them off and sealed the pin holes with silicone Not happy about it.
Not shown is the second piece of scrap noodle plastic on the sides of the battery box to hold it in place. BTW for the curious the black thimajig on the battery box lower right corner is a on/off 12v toggle switch recommended by Eagle. It has a waterproof cover you can buy for~ $1.50 And you will also note four wires into the box. I used miniature rubber donuts to keep the holes small. Two wires are for the FF. The other two for the compass which is rigged parallel circuit wise to provide for it's backlight LED.
Third idea would be to use some sort of mechanical device around the scupper hole posts. A simple line tied tight might let you attach other objects.
But I'm hoping the poly glue works. All my previous efforts to keep velcro holders down have ended in failure. If the poly glue works velcro will be a very welcome addition to securing objects inside. As will the ability to glue down eyelets.