@jtampakes I drilled a hole on the underside of the top about have the depth of the top thickness. I use a really small threaded nipple just enough to stick up above the coupling but hold the top down securely. I used liquid nails construction adhesive in the hole and then just tapped the top down with a rubber mallet. The hole size was perfect because the threaded nipple was really snug inside the hole. The underside of the top is resting on the union, so it is very solid. I added 2 new images to the album to try and help explain what I did.
https://imgur.com/a/EeMNwcr
https://imgur.com/a/EeMNwcr