Access to new under floor tanks.

I am currently in the process of refurbishing a 5 meter Pro Trout boat and was hoping to get some advice about installing access to new under floor tanks.
The boat deck is foam filled (around 15mm thick) and has two stringers running down the middle about 650mm wide. This is where I would like to install the tank. So far I have not taken to the deck with the saw.

I have spoken to a couple of people with a bit of know how and they seem to have different ideas. It would be nice to utilize the cut-out to save cost but if there is a better way then I would consider it.

My plan is
1. Remove the entire section of decking plus about 10mm into the support beams in which it is glued / laminated onto (about 650 x 750 mm).
2. Install the tank (I am ok with this)
3. Laminate a “sill” with glass (maybe angle?) about 10mm so the hatch lid can sit on top of the support beams underneath.
4. Build a new hatch lid with laminate and foam to fit into the sill.
5. Seal it with sikaflex and screw in. (might use something other then sikaflex)

If anyone has any other suggestions please let me know. Again, it would be nice to use the cut out lid but I can’t figure out how.

Picture of deck:




Thanks,

Geoff.