Hi JZ, I did this a couple of years back. I used a Vetus Universal Inspection Port, it's suitable for diesel.
www.keoghsmarine.com.au/vetus-all-purpose-tank-universal-inspection-port-suits-all-sizes-of-atank-ilt120

Whatever you do, don't buy the Vetus holesaw that is supposed to be ok for this, it's not. Have a look at the first pic, that was taken after a good long session trying to get one to cut, it didn't, just polished the SS. I sent it back and got a refund.

The port itself is a good product and I'm happy with it. My tank had poor access inside a cupboard under the galley sink, so cutting the SS was a PITA with a jigsaw. Looks like you've got easier access there, so a jigsaw with a SS cutting blade will make short work of it.

The only issue you might have is with the height of the port, it doesn't look like you've got much room to work with there? hole in the lid so the port protrudes into it maybe?

Great information. I have a fuel tank that needs an inspection port and this looks like a great option. I am a bit concerned with getting all the metal filings out for a clean tank though. How was this done?

Don't use a standard plastic round hatch, as the O-ring may not be suitable for hydrocarbons. I have one on the keel water tank, and when there was a bit of an oil leak onto it the o-ring swelled up and had to be replaced (and the leak fixed of course).

Cheers, Graeme

Hi guys, thanks for all the pictures and links shared. I'll consider my options. Cheers

Ok thanks. My tank had the bug and I've removed it so will cut the hole, clean it out then fit the Vetus option.

They updated their website. The new link with diesel tank inspection hatches is: www.cleandiesel.com.au/product/190-mm-round-diesel-tank-inspection-hatch-150-series/

Definitely find a plastic that's compatible with diesel. They tend to get brittle over time.