Bob see if you can source some liquid bitumen, as they sometimes use
for bitumen{Asphalt} repairs to seal around the edge of road repairs.

If you heat this up, to make it runny, pour that in; when set will be
firm with some flexibility..& of course its waterproof.
I have a can from when I worked for Shell, but guess is available at
merchants{Lumber Yards}.
You could also try bitumen mastic as used for fixing roof
felt/shingles; guess easier to obtain that road bitumen.

When I use that I heat up with paint stripper gun{Gas or electric}; be
aware that it may flame up, but only temporarily, till you remove heat.
And of course it sets instantly.

So have some sand around to smother if you have to.