The foam you need is plastazote LD 45. This is firm enough to hold a strong shape that a latp weapon or shield needs without being to dense. While there are certainly other suitable materials out there, any kind of expanding foam or Karrimat foam is not it.
A decent larp shield will be two or preferably three layers of foam.
It normally comes in sheets 1000mm x 1500m so depending on the size of your shield you might get away with just one sheet but you might need two.
Glue it together with evostick, (solvent based if you can get it) and fashion a grip from either wooden downing (20mm thick at least) and either cover with cotton canvas and paint or latex and paint.
It is definitely possible to make a good professional looking shield for under £50, but that assumes no wastage, which when doing anything for the first time is quite an assumption.
Apologies if I’ve shot ahead and answered questions that you didn’t actually ask…got a bit carried away. Take anything I say with a pinch of salt, it’s been a few years since I made shields.
T