Take a piece of good quality, thick paper. Tape it to the top (bottom, but you've got the amp upside down on the bench) of the amp with masking tape, in at least 6 places pulling it tight. Take a childs crayon and rub along the edges to mark the outline. Then find the mounting holes and mark them with a felt pen (use a big  + and o superimposed).  Do this just before actually drilling the holes, paper stretches quite a lot in damp weather (not that you'd get much of that this time of year in Spain.....) so don't leave it lying around.

Cut round the outline and tape it to the top of the amp (you might need to remove the handle) and use it to drill the holes. Personally I would drill smaller pilot hole first to prevent damage to the tolex, then use a modelling knife to cut the tolex before drilling the correct size holes.

You're welcome. Enjoy the amp.