I have the same problem with my PRS Mccarty 594. My Fender strat is so easy to hold, the weight balances itsef. While the Mccarty slips and the weight does not balance. The strap does helps, but I suspect that I would get better result with a leather one. In the mean time, a cheap kitchen non-slip mat from the dollar store under my leg did the job.
I also have an eye on the GuitarSlipNoMore thing which look more convenient/aesthetic. But, with the shipping and conversion rate to my country, I am hesitating since I can’t try it in store.
Another vote for a strap, although, I see from your comments, you’ve likely already purchased one
I have my strap set so that the guitar rests on my leg, but only just. I find this length works well for me also when standing, so if I play in either position it feels pretty much the same. I also always use a strap as I’m a klutz and would 100% drop it on the floor if I played without one whilst sitting
I like the Fender “Super Soft” cotton straps for a no slip option. I regularly see them on sale and they make a good add on item when bargaining on new gear as they are not too expensive.
