I have injected thru a loose weave shirt when at restarants. I make sure the shirt is a dark color in case it does bleed. I do it under the table so nobody sees and flips out. I don't do it often but I have never had trouble.
Usually I inject 10 units of insulin per meal. If I run out of insulin, I use 2-3 tables of metformin instead. I do find it effective but I also find that it messes with my stomach.