I would go with West System 105. It is for boat building so it will hold up.
Get out any loose gunk in the voids but you dont need to worry too much about making it pretty. With the large voids you only need to get a seal then build on it with subsequent pours. I use masking tape then lay it on grease proof paper because you will get some leakage. Buy a sheet of cheap particle board to lay it all out on. Peel off tape just after it has set. You can sand back anything that stuck and recoat it.
Get the bottom sealed then work out what you want to do. You may need to turn wood on different angles each sealing pour. Buy a small butane torch to get the air bubbles out, but be careful you don't cook resin if a bubble wont shift.
With those large voids you could put anything in them, from a red back spider to wild flowers or pebbles. You are only limited by your imagination.
As yours is exterior I would only resin the voids and deck oil(or similar) the wood. Resin is high gloss finish and shows any scratching etc.
Good luck
This one is all resin
This one is resin edges on oiled top.
And buy the measuring pumps for the resin containers.
Cheers Glen