Here I'll show you how with some extra work you can get a good looking weapon, not just a functional one.
Advanced soft larp knife
Materials: pair of flip flops- I picked biggest available, made from a sturdier foam. Yeah I had to buy them, I don't have that many old pairs just lying around. Various tapes, glue, a bit of pipe foam, a strip of leather - got it from an old fake leather belt - you can improvise here. Glue.
Tools: scissors, paper knife.
Recommended for: A hidden blade, backup or a boot knife. Since it lacks a hard core don't try to use it in open fights as a parrying dagger unless you don't care much about your knuckles.Depending(literally) on the density of the flip flops you pick for the core, it can fly quite well - at least the one I made now is just heavy enough to fly very decently as a throwing knife with range of 5-10m.
And here we go.
In the end you'd get something like this if you were doing ok. Thats why the pommel was left barely hanging.
we are done. The only thing I could have done better was the sharpening of the blade, but I do have some ideas how to do that part better.
This is roughly as far as you can get without latex, but unlike latex weapons this takes 15 min. to make so its eventual loss would hardly be mourned.
And it certainly does not compare to these: