Look at maybe some downpipe for the speargun, I don't have one but there could be some brackets made to hold it up under the gunnel along a pocket or something.
With the esky, I'd go for something flush if you can, so when it's not there, there isn't anything to stand on. Maybe there is way to clip/strap it back to the centre cab.
Maybe chuck some photo's up of the boat so we can see what we've got to suggest ideas. Photo's with and without the esky would be good.
Corry