Seems like this is getting over-engineered.
KnotYet made a great suggestion to use velcro. The stuff you want is industrial grade or heavy duty stuff. Both sides have the plastic hook stuff, there is no “fuzzy” side. It has serious peel-and-stick adhesive on the back, so you don’t need any epoxy or other additional adhesive. Clean the box and the transom surface and mount with the velcro tape. It will be there forever unless you want/need to remove, which will still be possible. You can still pull the box and reattach if needed. There is no need, IMO, for plywood or other mounting block. Look for 3M or Scotch brand on the velcro.
Another option for this type of thing is 3M VHB tape. Look it up if not familiar. It will mount that box with no penetrations or mixing stuff and the bond will way more strength than necessary for this application. This won’t remove without destroying the tape, but way easier than epoxy. The tape seems expensive for tape but is cheap for its capability.
A few more suggestions for future use:
1. 5200 has its place, but is very slow to cure (days) and is messy, expensive per use, and is much better suited to other uses. Almost always, 4200 is a better choice, but that debate can happen in another thread. Neither product is needed for this project.
2. When you do need a small amount of thickened epoxy, such as to mount a small block, its quick and easy to use a two-part kit from the hardware store. The cost and quantities are small and you can get it with thickener already in it. Its easy to store and easy to use. I have gallons of epoxy and multiple thickeners but its way too much trouble and mess for a very small job.
3. Penetrating epoxy and thinned epoxy for a first coat is a popular myth. The waterproofing comes from the epoxy layer, and thinning it just means you have to use more coats. Epoxy has tremendous bonding strength to a properly prepared surface so doesn’t need to “penetrate” for bonding. The penetrating aspect is for marketing, not because it is better. Skeptics should read manufacturers recommendations. They know how to best use their own products.
Now go mount that box with industrial strength velcro.