A 9x13 pan will give you 117 square inches; half of which is 58.7 sq inches. Take the square root of this and divide by pi, and you get 4.32", which would be the radius of a round pan with the same area. Therefore, you should use two 8.5" to 9" diameter pans, or one 8.5" and one 9" diameter pan.

OK, I understand. I have those. Now what kind of throw-away pan is best to bake half in?

https://sallysbakingaddiction.com/cake-pan-sizes/

Disposable cake pans come in common standard sizes.

ETA, from the linked site:

9×13 inch pan holds 14-16 cups of batter, essentially the same as 2 9×2-inch round pans.

Are you asking about what the pan is made of? I know there are paper and aluminum pans, but I've only used aluminum disposables. I believe there are plastic disposables as well, but I have no experience with those.

Not sure the material of the pan makes a huge difference in the results as long as you follow directions...

Wouldnt the quality of baking be affected by a thin aluminum baking pan? I have some nice cake pans and I cant imagine getting the same results from thin material. My cake pans are good ones and give good results.

Maybe, also, it depends on what type of cake you are baking .One of those super moist cakes like a carrot cake, for instance, might be fine baked like that.

I used to pick up pie pans for really cheap at yard sales and would keep a couple of them around to use instead of the aluminum ones when I had to take them some where. I would just leave the pie pan and let it be kept or discarded.

If you take a step back from your plan, do you really want to give a gift cake still in its baking pan? How about making your usual cake, cutting it in half, decorating it, and offering one half on a pretty cake board? You could cut all round to avoid having one edge different from the others.

The nice thing about disposable pans is that many come with lids, making things easier to transport.

When using disposable aluminum pans, I place them on a solid cookie sheet rather than directly on the oven rack because the thin aluminum needs help from the solid surfacae getting the heat evenly distributed across the bottom of the pan. I discovered this when baking several coffee cakes for a group and the centers tested done but were actually underdone when the rest of the coffee cake was fully-cooked.

No throw away bakes well....better to either remove from he pan or buy several thrift store heavier pans o give with a cake.

I ordered some Pantainers from Amazon - as recommended by Beesneeds. They have arrived and I will see how they work.

Beesneeds, when you use these aluminum throw-away pans do you put them on a sheet pan while baking - like aliceinmd?