Ideas for a gf sauce for fish pie please

How about making a sauce with cornflour? I made a parsley sauce with cornflour, to have with gammon, not long ago. It tasted fine. Would do the same if making my usual fish pie, if there was a GF guest coming.
I did google 'white sauce with cornflour' for a recipe.

www.jamieoliver.com/recipes/fish-recipes/fish-pie/

My fave recipe but I don’t use the chilli.

To thicken sauces I bought some peasemeal from Golspie Mill, Sutherland (I think they'll do mail order).

It's a bit khaki in colour, being peas, but it thickens well and is tasty.

Make your white sauce, add mustard powder and gratedcheese.

You can also use the poaching liquid (half and half with milk) and an added fish stock cube to give it more flavour.

I just make a basic cheese sauce using cornflour.

My mother always made a white sauce with cornflour, either sweet or savoury. You could try making your own cheese sauce with it.
Have a trial run first to make sure it tastes good.

Would using potato flour/starch to thicken the sauce work for you? then just adapt any recipe.

Crême fraiche with grated cheese and seasoned to taste?