Here's a coupla dumb bung ideas:(untried)

I assume:

- a flush hatch with a 1/4" gap all around. (for ease of description)

- a flange 1/4" lower than the hatch lid and 1" wide all around.

- gasket material only 1/2" wide on flange (or hatch preferably), so that there is a 1/2" gap left over between the gasket location and the edge of the strips all around.

- the result is that when you put the hatch in place there is a 1/4" gap all around with a further space 1/4" high by 1/2" wide directly below.

1) Wun Dumb Bung:

Take 1/4" round tubing at 1/2 the circumference of the hatch and cut longitudinally down the middle so that you get a full circumference of c-section tubing. Cut this in 1" long lengths.

Alternately glue the 1" lengths to the hatch and the flange so that they interlock to form a cove all around the gap facing the strips. You now still have the upper 1/4"x1/4" gap, but now only have a 1/4"x1/4" clear gap below with the open c-section facing the lower one horizontally.

In the hatch, cut a notch like the one in the pull start of your granma's lawnmower or old small outboard motor.

Take 1/4" or 3/16" bungee about 20% less than the circum. and glue or tie one end to one hatch c-section directly below the notch and tie a figure 8 knot in the other end.

Place hatch in place, wind the bungee around like you're about to start the motor, pull knot in the notch, and yer done!\

You know have a concealed attachment, slightly large gap, with only the knot showing.

Other considerations:

- use webbing and thin the gap.

- anything that makes a cove can be substituted for the tubing. For example it would be way better if the c-sections were formed into the flange and the hatch.

- the main drawback as I see it, would be putting pressure on the gasket. But a two part would be ok, or tight proximity silicone gasket would prob. be o.k.

2) Too Dumb Bung

Form a continuous rigid cove all around the flange in exactly the same location as in part one. (i.e. no interlocking, it is in the flange only)

Attach a 1/2" wide x circumference strip of rubber to the vertical edge of the hatch (i.e. it hangs down 1/4" all around).

Attach one end of the bungee to the flange this time, and wrap around like starting the motor as before. This time you get a second seal.

Well, I hope this hasn't wasted all your time, but I think there may be something to the many variations of this approach.

Anyway, see ya later.