In that case cut the corners using a hole saw in a drill for the corners (if round corners are OK) then do the straight cuts from the underside using circular saw with a fine tooth blade OR do the straight cuts from the underside with the circular saw as above, flip the worktop over and carefully saw the corners out from the top using a brand new reasonable quality fine tooth (9 or 10pt) hard point crosscut saw (something like a Bahco Barracuda 244+).
The problem with using jigsaws to cut thick materials is that they rarely cut square (the blades tend to flex, especially those crappy T101BR downcut blades so often, and mistakenly, recommended for this task). Even the industrial models are rarely brilliant (there are exceptions - the Festool Carvex is OK-ish, whilst the Mafell, P1cc is exceptional when combined with their extra thick W1 blade, but so it should be at circa £500 for the saw and £30-odd for the blade). Jigsaws all work better if used with an upcut (standard) blade from the underside of the material, but as soon as you go round a cornwr with a jigsaw blade it takes on set to one side and will no longer cut square, so if you do get a jigsaw, drill out the corners or use square corners to avoid messing up your blade
