2mm ali sheet sourced readily enough (hole just under 300mm diam, so precut sheets of that minor dimension were OK). How to cut a neat circle, though? At 2mm my hand nibbler was a bit strained, and doing 1m of cutting (300mm x pi, close enuff) wasn't a fun idea. 12in hacksaw round each of 4 corners would've been possible, but hard to follow the curve precisely. So... though my jigsaw's just a B&D cheapie bought 10? years ago, I checked through the blade collection to find a couple of 'Piranha' fine-tooth metal-cutting blades. Applied to the inner edge of the circle drawn round the perforated sheet as resting on the plate - turned out to be very controllable, no juddering with the plate clamped to the workmate, easy to follow the line. (Sheet came with disposable white placcy covering, so markout and soleplate marks didn't end up on the ali surface).
Result of less than 5 mins cutting - a nice neat circle, fitting perfectly; edges rounded off gently on the (Ferm cheapie) bench grinder.
I guess I hit pretty much the sweet spot for jigsaw use - cutting curves in thin sheet. While for 18mm ply I'd always bring out the circular saw, for thin sheet like this - MDF, hardboard, and as I now know ali - the 'consumer-grade' jigsaw's just fine, as the inevitable blade-bend doesn't affect the cut on so thin a sheet.
As it happens the nibbler had been in use on a 1.2mm bit of ali sheet earlier in the day - cutting the verge for the first time, the mower (rotary, petrol, with plastick/GRP deck) unerringly found a small discarded lorry-tie-down strap, which wrapped round the blade with the ratchety bit smashing into said deck. Chunks of deck came away before the motor stalled. So, I added to the collection of ali patches which were already in place on the other side of the deck with one underneath/inside, held in place with a few pop rivets. The mower was a bit of a distress purchase anyway, and is in its 5th year of cutting: bought after moving to a place with a far bigger, and somewhat rougher, garden: a 'big-blade' electric Flymo self-destructed within a coupla months - instant refund from B&Q - and this one bought at a Local Shoppe for Local Lawnmowers in its stead. What the GRP buys you in corrosion resistance, you pay for in fragility - it really doesn't stand up to thumping from the kind of junk which ends up in the verge expecially. Yes, I usually try to clear up any nasties before mowing, and keep my eyes open; but expecially with over-long grass on a first cut, with the revs up full, any little lurkers end up thrown against the deck with some force...
Ah, the joys of spring... Stefek