Posting this on my phone as the home QTH has no Internet connectivity since Friday morning, which has resulted in no PMR446 gateway in Richmond, unfortunately this is BT's issue as opposed to mine.
I found a second nut and machine screw today while tidying up, needless to say this allowed me to drill a second hole and fasten the USB port to the tin lid properly, I removed the SDR to drill the hole as I did not want to damage it, it was returned to the tin when the second screw was tightened.
The SDR is currently tuned to GB3IR, the local 2m repeater, the antenna is a 2/70 antenna so fine for this use, performance appears to be good, the next stage will be sealant around any holes to prevent leaks, then some vegetable oil for coolant as far as I dare fill the can, as per the information on SDR for Mariners, and a final bead of silicone sealant on the outside of the lid to be absolutely sure.
This has been a slow project but it is almost there, a scanner antenna will be next for this, hopefully I can see what this cheap SDR, when surrounded by metal and immersed in oil, can really do.
73 de 26CT730
PS, I hope to have Internet restored and the gateway operational soon