I would split the project into two parts, the Fly Screen and the Tank Covers. Make up a three sided box from Hardboard or stiff cardboard large enough to fit the parts inside plus 50mm all round. Fill the boxes with expanding foam. Remove the Hardboard supports and start shaping. Even if it all goes horribly wrong all you have lost is some time and £10-£15 of materials.

Carbon Copies Ltd

Now that's a cracking idea, cheers Warren

These are really handy for this type of work and cheap as chips.

Carbon Copies Ltd

If you give me your email address I will send you pictures of parts that I have made,and explain I can't 

Sounds daft but where'd you buy something like that warren? Google isn't assisting.

eta finally found it.